At 17 years old, I would recommend you getting the following for the $250-$300 you have:
Craftsman or Jet 2X42 - the 3/4HP models can be found used between $100 and $250. They come in two motor sizes. The 3/4HP model is a good unit, but runs about $450 new. The 1/3 HP unit only costs $150 new, but is underpowered for knife work. You want to look for a 3/4HP on ebay, Craig's List, or your local trader paper. Most any brand of 2X42 with a 3/4HP motor should do fine.
Any left over money after the grinder should be spent on belts and sandpaper. To start, you want three coarse belts - 80 grit; six medium belts - 120 grit; three medium fine belts - 220 grit; and six fine belts - 400 grit.
Also get 3M Wet-or-Dry sanding sheets or Rhyno-wet brand sanding sheets. You want 220,400,800,1000,2000,2500 grit. It would be best to get a 10pak of each, but three sheets each is minimum.
A backup low budget plan is to look for a good deal on a 3/4HP 4X36 or 6X48 belt sander/grinder and run 2X36 or 2X48 belts on it. This won't be a perfect setup, but will work if funds don't allow more. These can often be found for less than $100. If you are fairly handy, you can easily modify one to have a 2" wide platen, but you can just track the belt left or right and it will work as is.
If you are shop savvy, you can convert one into a 2X72" belt grinder.
The alternative is to save longer, and make knives by hand sanding and filing for now. Once you have $500-$800 saved, you can build a EERF, NWG, or similar kit grinder for about that price with a little scrounging.
The Grinder-in-a-box from Polar Bear Forge is a good build kit. The full package with multi-platen is less than $800 complete. If you can scrounge up a proper motor, you can knock $200 off that. Anyone with a wrench set and some shop experience can build it.
.
