If you aren't comfortable placing your own car seat in the car, then I would recommend you rent one with the vehicle. We've done this on numerous occassions and haven't experienced any problems at all.
If hygiene is an issue for you then just place a wrap over the sides?
AFAIK the reason for keeping a child rear facing until the age of one is to do with the strength of their necks should there be a crash. Their necks are deemed strong enough to withstand a head on over the age of one.
Good luck with it all.