I suggest starting out with a basic 2nd hand machine in good working order. They are cheap, return on investment is quick. Once you learn the basic skills, you can upgrade in whatever direction you want.
My 1908 Singer 28K is a very capable of producing a well sewn garment. However, there are a few things that make things much easier.
1) Reverse stitch to lock the stitch ends. A little button to reverse is nice to have, most machines post WW2 have something to do this. With the 28K, one would lift the foot, pull the fabric back a stitch or two, drop the foot and cotinue sewing. Not difficult, but you'd be surprised how many times you need to do it!
2) A motor. This allows both hands to be used to guide the fabric. A treadle would work, but is expensive, hard to find and is really heavy.
3) Zig-zag stitch. Fabrics have evolved and we now have a selection of stretch fabrics to choose from. A zig-zag stitch stretches with the fabric, resulting in a stronger seam. Some of my machines have loads of fancy stitches, but I have never used them in any clothing patterns. The zig-zag stitch is required in a number of patterns I have.
If you are thinking of buying a brand new machine, almost all of them will do a decent job on clothing, for a while. If you want it to last, you need to ensure it has a metal frame underneath all the plastic.
Take a look in the "Machine Talk" section for reviews and recommedations on specific makes/models.