You might be able to rent with a 300-point bonus like the birthday or anniversary PCs. That would get you there fastest, and perhaps cheaper than buying them. I can't remember if they require 2-day keeps or what, but they require renting a full-size car on a weekend (or weekly).

Buying 350 costs $70 now, as you said.

If you're only going to save $75 anyway (inferred from saying you'll only save 5 bucks if you buy the points), I wouldn't bother. Just rent paid, and save the points for later. That sounds like you're only taking a weekend award or a couple of free days, which is not the best use typically. Weeklies where you're wanting the car for 6-7 days tends to be the best value for points. (Ex: weekend non-peak is 2000 points, weekly non-peak is only 2500 which gives you up to 3 extra days for just 500 points.)