A reasonable guess would be 35 watts.   Standard wisdom would suggest you need 70 watts worth of speakers.   The H30/12M combination would a gamble.   You might get away with it for a long time.    I am sure you could find guys who have experienced both extremes of success/failure with that amp and speaker set up.    I tend to prefer Eminence speakers for most applications.
Regards the V-30..... harshness, high-mid hump, mid spike, and other such phrases are very common.    But the V-30 is also one of the best selling guitar speakers of all time.    A lot of recorded music (that we all listen to and like) was made through the V-30.    You will just have to buy some speakers and hear for yourself.
Series or parallel is really up to you, and dependent on your OT impedance selector.
Good Luck

AFAIK, Ceriatone is the only one that makes cabs for their amps.   So yes, you will have to make/have somebody make a custom cab for you.
Good Luck

Funny, I never even thought of that.   Do you have a return or something?
Maybe email Nik.   He has a lot of stuff going in and out of his shop.   Perhaps he could point you in the right direction.
Good Luck

Looks good, well done!!!
Nik sure does supply a nice product.   Not much to complain about really.   For some reason, I don't care much for the Alpha pots.   I have never had a problem with them, so my suspicion is unfounded at this point.   The guts look pretty much like a CTS to me.   Anyway......
My vote would be for a half stack (mini).   I built the BFVC in a head.   I am not joking when I say it is one of my favorite amps, and we have all kinds of amps.   There are power limitations of course, but all amps have that in one form or another.   So I cast another vote for a head, and another vote..........
Good Luck

Marshall did make a Lead/Bass combo.   That guy from The Jam uses them.   What is his name... Paul Weller?   Is that what you have in a head?   If you do not know for sure, find out.   They are fairly rare amps.   Any Marshall from that era (1975) will always be worth some money.    Is there any way you can keep it, and buy the Ceriatone?    Ceriatone makes great, hand wired amps.   We have several, but their resale value is low.   If you can find one used, you will be miles ahead.
We have both the 1986 and the 1987.   The only question is the OT.   They are very good quality transformers.   The only question is how close they are in spec to a plexi panel Marshall of that period.   Other than that, your ears and fingers will think you are plugged into a Marshall.
Good Luck

Are these the amps that Keith Richards uses?   Tweed Twin,  High Power Tweed,  Tweed Super Twin...   I'm not sure what to call them.
Thanks

The wire from the OT is negative feedback for your Presence control.

I imagine that is a dropping resistor on that 32uf can.

Not sure about the others.   The two on the rectifier have a uf capacitance, but they make the component look like a diode.
I imagine the cap and resistor on the SB Switch is some kind of snub; to prevent the pooping that sometimes occurs when switching the HT.
My experience with Nik is that he is pretty fast with emails.   I encourage you to ask, he is very helpful.
Good Luck

It is so clean, and I mean literally.   What do you use on all the solder joints?   Very nice, looks like it went through a freon tank.   If you do get any extra hum, you may want to chop stick/shorten, re-route that lone, green, ground wire from your (ground bar maybe?) to the tranny bolt.   Sometimes a little extra wire over there makes a lot of noise.
I see you have faith in CC resistors.   Have you ever read Sozos site in regards to the foil line on their coupling caps?   I am not an experienced builder, but I think you did a great job.   Your component placement is superb.   You sure you didn't have your teenage daughter do that part? 
 Well Done!!!!

Does it switch a bright cap?    I don't know anything about Matchless amps.   What does it sound like?   I would think if the push-pull by-passed the tone tone stack, wouldn't you be able to tell if the tone stack didn't work?
Maybe see what Matchless has on their site.
Email Ceriatone.
Good Luck

Does one amp have a choke, and the other does not?

There's nothing stupid about any of your questions, they are all relevant.    Just took a quick peek at the layout; it is a cat biased amp.
OK, I have a stupid question.   I know absolutely nothing about these amps.   What is the difference between a Marshall 18 and a Marshall 20 watt amp?   I suppose I should just look at a few schematics.....
Perhaps they are just two completely different animals from the tone stack to the out tranny???
Thank You   

Is there something specific you are wondering about?

1.  Power Scaling.   Check out London Power (Kevin O'Connor).  Also take a look at Brown Note.

2.   Not sure, you need somebody smarter than me.   I think the problem might be that the suppressor is tied to the cathode in a EL84, where as in a EL34 the suppressor has its own pin.   I think that is where they manipulate the power.   If you email Nik, I am sure he will give you the real explanation.

Good Luck

Are you asking about setting the bias for a 50 watt plexi, 2 x EL34, one of Ceriatones kits?   
Have you built it yet?
Does Nik/do you use a 1 ohm resistor from cathode to ground?
Assuming the amp is built correctly, the bias pot is where it should be, and you can safely put your hands into 450 volts...
Check the plate current from ground to the 1 ohm resistor on Pin-8.   Lets assume a reading of 35ma (will be 35mv on your DC meter).   Now check the plate voltage on Pin-3.  From ground to Pin-3, Volts DC.  Assume a reading of 400 VDC.   Multiply the two numbers = 14 watts
I believe the EL34 is a 25 watt tube.   Assuming a safe dissipation of 70%, 17 watts would be your "maximum" desired output.   With the previous assumptions, you are at a very safe 14 watts.   You can raise the current, but at this point you should start listening with your ears and fingers to decide what sounds/plays best for you.   We will assume Nirvana lies somewhere between the 14-17 watt mark.   I imagine there is some cathode current to consider, but at this point you may just want to ignore it as a built in safety factor.
I have given you a guide, not a Bible.  This is the INTERNET.   You have no idea who I am, or what my qualifications are.   Keep that in mind.   Everything I have written could be erroneous.   Be smart, check another source.
Good Luck

Keep in mind that your term "half volume" is a misnomer.   It is a half "power" switch, and that does not compute to anything near half "volume".    If you are playing a venue that is too small for a 100 watt head, you may not get much relief from 50 watts.   Just something to consider.
Good Luck