I know you are right Brian, but there are other factors here. Since I will be looking for show quality finishes I expect that I will have to dress the welds and maybe even use some filler before painting.
I once worked with a welder that was very skilled and did die repair. The man was an absolute artist with TIG, even so the welds still needed to be machined afterward. If you want welds good enough for show, you will likely need need to clean them up no matter what welder you use. It is just a matter of what is acceptable for each weld.
Also, I would like to have only one welder, if possible, and would like to be able to weld aluminum and some stainless.
I'd like to have only one router or only one bandsaw but the reality is that sometimes you are better off with multiple machines. A really good multiprocess welder will be expensive so the trick is to find a lesser machine that does most of what you want.
Doing aluminum puts you into a more expensive machine. One rule of thumb that I've heard claimed by some welders is that you need 1 amp per thousands of thickness for aluminum. That can put you into an expensive welder. The other thing with aluminum is that you loose a significant amount of strength in welded aluminum, it is often better to rivet aluminum
I don't know if the above is possible.
Nothing you asked for is impossible. There are many multiprocess welders out there but a welder that does everything you want will be expensive. Well expensive relative to your budget. This especially if you want a TIG welder that can do aluminum well which demands AC TIG capability.
Honestly, if you haven't welded before consider taking a welding course that a local school might offer. You of course would learn a bit about welding but more importantly you will get a feel for the processes.
Another thing to consider is your age, having recently blown by 55 I can attest to the fact that old age does impact your welding. Wearing bifocals for example can be a real pain when it comes to keeping focus on the weld puddle and seam. In my case I'm no longer as steady and sure as I was years ago and this does impact weld quality.
A welder isn't the only equipment you will need. For example a 4.5" right angle grinder is a requirement and you will likely want multiple grinders. A nice collection of clamps and welding specific hand tools are also required. A die grinder comes in handy too. Depending upon what you already own this stuff could add a couple of hundred to your start up costs.
Look into your houses electrical system. My house is very old, built in the early 1950's and has a pathetic electrical service, this is one reason why I went the MIG welder route as it won't overload the electrical service. Some of the transformer based welders can draw a surprising amount of current requiring a dedicated line from your panel box. Setting up an outlet for your welder can get to be expensive. The new inverter based welders can be a big advantage here though. Not to mention the inverter based welders are often one man movable.
Welding can be very dangerous to your health!! Do read up on the health dangers. Your shop may need additional ventilation or an air cleaner which is another expense. Opening a garage door can do wonders except for processes using shielding gasses.
Hope this helps.