Ah, you're onto something I've done a lot of. One of my best mates writes musical theatre and part of the reason I got involved in home recording was trying to help him put together CDs he could send off along with copies of his scripts. I must have done something right...a few of his things have been produced, albeit nothing on a huge scale yet.
Anyhow, I'm sure this is one of those topics where there is no single right answer but my suggestions would be:
First, don't get too simple on the MIDI keyboard. Give them enough sounds to get an idea of how the eventual arrangements would sound--which takes you into the realms of sequencers and nice sounding samples/instruments.
Second, depending on what style of songs you'll have, your tracks will likely be a mix of a few soloists plus some choruses. Choruses will be difficult to handle "for real" in a home studio scenario. However, with a couple of mics you can probably do things a track or two at a time, then record a small number of people (you mention six--that was about my maximum) recorded several times harmonising with themselves. Then it becomes a mixing job!
So...suggestions.
My composer friend works in Sonar so I have a copy of that. However, my background is pure audio and I like working in Audition. My solution was to receive Sonar files from my friend, render the tracks (usually 12-20 in number) to WAVE files then add the vocals and mix in Audition. However, this might be a bit cumbersome and you might well find you could do all your work, MIDI and WAVE in Sonar--or perhaps a cheaper alternative like Reaper.
Mics? I actually used a mix of small and large diaphragm condensers--most vocals solos on LDC and chorus stuff on SDS in an X-Y configuration. However, that was because that was what I had...if I was buying from scratch I'd go with two decent LDCs.
You'll need an interface. There are lots of interface threads--just go for one with at least two mic pre amps and phantom power on both.
Something you haven't thought of--you'll need a way to provide a headphone mix to everyone singing--so budget for a headphone amp with as many outputs as you will want singers. I got round a lack of headphones by telling singers to bring their own--a visit to the discount electronics shop for a handful of mini jack to quarter inch stereo adaptors and a few headphone extension cables and you're ready.
As with any home recorder, don't skimp on monitor speakers. You need to be sure your mixes will sound good to everyone you send them to.
Somebody will almost certainly pop in saying "yeah, but what about acoustic treatment?". And they're right--but don't worry too much. Theatres are fairly reverberant spaces so musicals sound good in that sort of place. As long as you have a fair bit of soft furnishing in the room you record in then add a theatre level of reverb to your mix--more than a typical commercial track--you can get away with it. (Now I cringe and wait for the calls of "heresy!")
Just to put my money where my mouth is, here's a couple of tracks done for my mate working as described:
Red Stars
Ballad of Annie Jane Wilder
Both were done in my untreated living room with no more than 4 people in the room at the same time.
Bob
