Power, weight, stroke, chassis/suspension type, rear gear, what kind of racing (you want to go fast, consistent, hit the pro tree, etc) quality of track prep you usually see,...

Transmission choice will determine your options and the costs. You can get a variety of gear sets for 3 speeds, even use 2nd gear to start. Glides have fewer choices but they are usually right for many applications. They top out at 1.96 for a decent gear set with 1.80 being popular unless you're making a ton of power and/or real light. Tire size matters but usually just knowing big tire ~32" or small tire ~ 28" will get you close.