i would spend 30 minutes trying to figure it out, and if i couldn't ID the problem in that time i'd replace it. at that age, i wouldn't want to invest too much time or money fixing it. it's at its end of life so anything invested in it won't take you much farther. so if you can replace a belt or a sensor quickly and cheaply, go for it. i wouldn't call a repairman, you'll be out $150 before he/she ever walks in the door. put that money towards a new dryer.
17 years is a good run for a dryer - in fact it's likely older than 17 years since it probably wasn't new when you got it.