You're in a tough spot I think and hope someone with more industry experience will help out here.
Aluminum... I don't think anyone will disagree too much that TIG is the way to go for small to medium aluminum jobs. I don't believe it can be done with OA (but not quite sure). It can be done with a MIG unit if it has high frequency capability. For small things with MIG, folks use an adapter gun that has a small spool on it. You need gas with MIG/Aluminum but can use flux core for steel (don't know about Stainless). I'm told however, that MIG aluminum is really for larger pieces.
Small pieces of steel is easily handled with TIG. It's all I use for small steel and even big pieces up to 3/4" (with proper beveling) if there's not a whole lot of welding to do. If I have many pieces of big stuff to work on, Arc/Stick is the way to go.
Brass? I know it can be done with TIG -and very efficiently too. It's also great on Stainless. I don't know about other methods for brass and I personally have never TIG'd brass. I've done lots of Stainless and it comes out beautiful but you need to watch the heat input. I use a secondary pulse function on stainless.
Brazing... I'm terrible at it but on the rare occasions I need it, I run propane and oxygen through the OA torches. -Uses the same connector fittings. You'll get nowhere with propane alone.
TIG is a little costly and takes some practice. I had a good handle on Arc welding. I was taught (long time ago) how to OA weld -but didn't do too much. I picked-up TIG in a few days and was making trustworthy welds after the 1st week. They continue to get better looking. TIG is probably the most expensive because of the gas (Argon and/or Argon+Helium for aluminum and steel does fine with pure Argon). There's an initial set of expenses for shielding cups, electrodes (use the lanthalated -and forget ceriated), bottle regulators etc. Once you buy all the packs of what you need, it lasts a long time. I rent/buy the larger bottles of Argon (300 cuft) a refill cost $65 and it's good for about 30-60 hours of actual weld time (varies on required flow rate). 10% helium mix really makes aluminum look beautiful and increases heat w/o eating-up the electrodes. I use it sparingly because helium shot-up in price lately to about $200/bottle. -Used to cost 75! TIG has a bit of a learning curve and takes a while to get proficient but I think it's extremely verstatile, outstanding for precision work and decent for low-production medium jobs. The strength of welds is outstanding.
If you think you'll be doing mainly precision type work, I think TIG is king. Even good for medium jobs of heavier stuff but it is a little slower process like OA welding. If I were a gunsmith like you, TIG! For brazing, you need a gas setup. Propane/Oxy is cheap and you can BBQ while you're at it.
Check this out. Half tempted to get one as a standby unit for my 205Amp unit. I have the second unit.
http://www.everlastgenerators.com/PowerPro-164-380-pd.html
http://www.everlastgenerators.com/PowerPro-205-379-pd.html
They also have a 256Amp unit... It's overkill unless you go with a water cooled torch. I rarely go over 160 Amps and can do it all day long with an air cooled torch. Above 180Amps, things get hot -real hot! You need water cooled stuff. BTW: Aluminum requires the highest amperage as it dissipates heat as fast as you can apply it.
Ray
it will be for welding /brazing. steel, brass and aluminum.
no cutting at all, nothing really thick either. maybe 1/4" steel at the thickest.
