I don't quite follow this statement - do you mean that certain schools will be able to provide proper, four seater planes while others only 2-seater and I should not even bother with those?
The single seat aeroplanes I've flown all seemed entirely "proper" as well.
There is no real difference between the skills required to fly a single, 2, 4 or for that matter 6 or 8 seater aeroplane. So long as you're flying something in good condition with a fairly standard set of instruments - just learn in something reasonably inexpensive.
Once you have your licence, if you want to convert to flying something else, just do that - so long as your flying skills are reasonably sharp, expect to take 60-90 minutes. You can also switch flying clubs whenever you wish, although unless they've mucked you about, I'd not recommend doing that during your PPL course.
I don't think that anybody would recommend doing the NPPL(SSEA) straight off; so, either do the JAR-PPL(SEP) or decide to fly microlights for now and do the NPPL(M). A microlight licence is much cheaper, but limits you to microlights (which don't have more than 2 seats) - a light aircraft (SEP) licence allows you to convert to microlights fairly cheaply and easily.
G