Heh no prob
If you look hard enough you can come across the Onyx's for decent amounts of money. But you are right, they can be a good chunk of change. PM me and ill give you a place to call that I go through and have good prices on what they carry, if not the most outgoing of places
If I remember right youll be able to get the mixer and card together for about the same price as the 1640 standalone most places, and before you ask they are legit, and only sell brand new out of the box stuff unless you specificly ask for something else, which they rarely have anyways
Fachiro if you want the offer is open for oyu as well
SM48? Well keep it around I suppose if you hit a jam, I am not to familiar with them but if it is rated lower than the 58s which it looks to be, I am not sure I would use it myself.
Acoustic Guitar recording many people go for a LDC(Large Diaphagm Condenser) mic, which being a condenser needs phantom power, so if you go this route make sure your interface or mixer or whatever can provide it, for any condenser mic you might use.
Not sure what you mean about using Cubase for sends, but for effects, yes that could work fine, in fact it is what many people do at every level of the game.
I suppose if you want suggestions for a combination of mics and techniques these would be mine...
Large Diaphragm Condenser, Audio Technica makes some supposedly decent cheaper ones, the AT2020, and the little more respectable AT4040 for more money. I havent heard about them used on guitar yet though, but they do a decent job on Vocals is what I am told at least, so they could double for recording Vocals.
Keyboard- Go straight into your interface from the Audio Out if you use a full digital piano. Otherwise record Midi like you do so far, recording an acoustical piano correctly is just plain a PAIN to do at times.
Your Sons Band-
Vocals- Shure SM58s for stage use. You could also concievably record their vocals on these as well if you needed to. The 58s are pretty well your standard stage vocal mic, again used in all levels of the field. If you get the AT mic from earlier I would reccomend using that for recording the vocals though.
Guitar and Bass Amps - Shure SM57s. Another standard for stage use, and very flexible, you will often find them used on all sorts of amps as well as snare drums and other things. Be careful though as you will probably need to roll off the bass on the EQ when you use these.
Drums - Pick up a drum mic kit. I would reccomend getting something a little better than the Shure PG series kit, but price is a limiting factor. But the advantage of picking up the kit is typically it makes it easier for you to place the mics as half the time they will have rim clips with them, and most advertise what mic to use where for a decent setup. Other than that there is the suggestions from earlier on recording with 3-4 mics that I use on stage a lot with good results.
Combining those mics with a board like the Onyx is what I would consider a basic setup for yourself and the band you described (Obviously speakers and amps help as well
that would allow you to both record demos and play live with reinforcement. Obviously you will be short an engineer, and please dont kid yourself, anyone can jump in and move things around, but it does take a bit of practice and a lot of work before you truly understand half of what is going on and get good at it. No offense intended here, again I am still not completly sure what type of experience you have with music technology but I am taking a guess
So if you want to learn this stuff feel free, but be ready to do a lot of research and reading to start to understand it and a lot more practice before you actually get good at it. I still dont consider myself all that good at it and I have been doing it for 10 years at least(Cant remember the exact number off the top of my head). I can give you some good resources to start reasearching though if you want.
Hope this helps some, feel free to post up more questions. Number one rule in anything recording or engineering related. If you dont know ASK, dont just guess until you have a very good idea of what you are doing, and even then it is better to ask first.