Noob needs crank replacement advice
 

I just became the owner of a used Cannondale R400 road bike. Tonight I noticed that one of my pedals was not sitting in the crank like it should and realized that the whole is stripped out to the point where there is almost no thread left. After much work I managed to get the pedal out but to me the right crank arm now looks unusable, which means that I need to replace it. I am not really at a point where I want to do a major overhaul on everything to jump up to a 10-speed. The current crank is a Shimano Sora triple, 30/42/52. Now I need to replace this pretty quick, and my question is, given that I have an 8 speed cassette, and an 8 speed chain is my only option trying to find another 8 speed tripple crank? I have been looking and all I can find is either a 9 speed or a 10 speed crank in the 30/42/52 configuration. Will any of these work? I do plan on a major upgrade in the future, but I want a quick fix now to get my pike back on the road.

I would appreciate any feedback on what my options are. If I need to expain anything better just let me know, thanks!

Just how unusable is it? If it's just the threads they can be restored/replaced with a wire-tread insert such as Helicoil.

Is that something a bike shop can help me with?

The crank arm you stripped, is it on the side with teeth & rings or the other side (non drive)? if on the none drive, could be a quick and cheap fix. ~$18 If on the side with chainrings, could cost you around $95 - a new sora part and a quick install at an LBS.

Sometimes you can rethread those cranks with a pedal tap. A bike shop should have one. I've restored some nasty cranks with these, they're wonderful. It'll save you some money.

I plan on a full upgrade to the bike but it will not be for a couple of weeks so I am looking for a cheap fix to hold me over.

Of course it would always be on the more expensive side. Isn't that how it always works?