With a 20k bank roll..... You can pick up a lot of machine with 15k. Lawn king is spot on about getting a good low hour used machine. Go ahead and go for a model with a quick attach front end loader, you'll kick yourself if you don't get the loader. The tread you will need will vary depending on what you end up doing with it. R4 tire tread is a fairly good all around tire, not very good in mud, not very hard on turf. As far as attachments.... start out with the basics. a good heavy built rotary brush cutter, a good rigid box blade with solid shank adjustable rippers, an angle blade and a finish mower. You can buy all of the other stuff as you have a need for it.
If you get out and hustle up work for that machine, you'll end up having to hire a guy just to run the tractor. I bought mine starting off just to cut fields and lawns that get out of control. Now I build gravel driveways, finish mow huge yards, cut fields, dig out ponds, till and plow gardens, level yards..... you get the idea.
by the way, you ever run a tractor before?