First, let me tell you that if you don't have the "gift" for sewing, classes & an expensive machine won't be worth anything. I had a P/T sewing business for years and made everything from costumes, dresses, drapes, pillows, anthing you can imagine, I've sewn it. My best friend saw me sewing all this stuff and she went out and bought a sewing machine thinking she'd just sew like I was....totally not the case. She hated it.
That being said, take it from a semi-professional in this arena, you only need a basic sewing machine. Find a reputable dealer in your area (or come to Kansas City and see/call the guys at Missouri Sewing Machine) and have them sell you a basic machine. You will never use monogramming or embroidery type stiching on the type of alterations work you have mentioned. I have used two sewing machines over the last 30 years. Both of them sewed forward, reverse & zig-zag stiching. That is ALL you need. If you forsee that you will do something that would need buttonholes, make sure your machine has that feature. Don't get dazzled by all the fancy stiches some machines make...you just don't use that for serious, non-crafty sewing. I had a Kenmore machine that I sewed on, until the reverse died, for about 20 years. I now have a basic Brother. The new machines will have electronic readouts, and that is OK.
Again, make sure sewing is your thing first...possibly borrow a machine from someone so you don't waste your money. I sewed actively until the adoption of my daughter 4 years ago. I really miss doing it. It was total relaxation for me. Best of luck and happy sewing.