you shouldnt need a prefessional to fit it once youve bought a seat. Parents should always be able to remove and check the fitting of a seat at any time.
You will be better trying to get an idea of the type of cars you will drive though and then check that the seat you buy will fit all. Dont buy an isofix unless you can fit it using belts as an alternative too.
i use a maxi cosi priori XP which fits fine into a vectra, ford c-max, mercedes and a range rover.
i think maxi-cosi have a site where you can check fittings, will go and have a look.