I currently brew in my garage but everything that I need is in the basement, so I was wondering if it would be reasonable to think about power venting the fumes, and therefore to CO outside to make it safe.
If you want to brew indoors you should seriously consider using electricity. If done correctly death by CO and fire are much less of an issue. There are lots of great options for electric brew setups (e.g. http://theelectricbrewery.com/)
Just make sure your wiring is up to spec and use GFI outlets to reduce the risk of death by electrocution. Brewing safety should be priority number one. As we all know that drinking a homebrew while brewing is an imperative leading to at least one stupid mistake per brew session. Making the mistakes less like to be fatal is a good thing.
In the winter I have brewed in an averaged size two car garage with the bay doors closed if it was really cold out. I use a two burner propane driven system and both burners run for most of the 4-5hour brewing session. I have never had a problem with CO. As a result I don't think you need to move much air. TO be safe I'd simply try and set up your indoor equipment near a door or window, and use a small fan for make up air.
A CO monitor nearby would help increase the safety level certainly.
While CO is a real concern not to be taken lightly, the reality is I don't think we really create enough of it with properly functioning burners to be a concern.
I've boiled many batches in our basement using a turkey-fryer style propane burner. I keep a CO alarm nearby and put a box or other fan near the stairs. I've never had the CO alarm go off, but our basement is approximately 800 sq-ft and our house (built in 40s) is most likely pretty poorly insulated. If you have a smaller basement or more airtight insulation then you may need to move more air. Definitely keep an alarm nearby and you'll probably need someone somewhere else in the house to hear it.
(disclaimer, I'm not a fireman nor an expert on CO, please don't die)
I have just looked up what the requirements for a gas powered cooking stove in germany are (quite common if you are heating with gas), and the regulations say that you need a room of at least 20m3 volume to be allowed to operate this and "sufficent" venting.
The biggest risk is the gas though, so I would suggest you install a gas alarm in the room. The CO2 is also dangerous, but you will notice CO2 very early (your body reacts to it, giving you the impression of bad air very early), gas might be odorless...