I like the idea of going down a folder / lomo and treating it as a bit of fun and as something for my son to get involved with too You're actually the second person this week to recommend getting a folding camera. The first recommended I get a Zeiss Ikon Nettar. I'll have to look it up!
Lefty
If you do decide to go medium format, i'd say avoid folders as i've not had great experiences with them (old dodgy bellows, dodgy shutters etc, and typically 6*9 format which isn't that useful).
I'd also caution against cheap Russian MF film cameras that you see on eBay. I've had a few, and every one has had problems of some sort - light leaks, faulty shutters etc. The Kiev 88 Hassleblad copy looks good, but is, IMHO, highly unreliable.
If you are just wanting to have a mess about, why not just get an old 35mm Nikon body to go with your existing lenses? I'd have thought that it would be very cost effective given the large number of bodies that were produced, and you can then shoot those older lenses in the way they were intended. If you are messing about, you don't need to go to medium format to have fun!
If you do decide to go medium format, i'd say avoid folders as i've not had great experiences with them (old dodgy bellows, dodgy shutters etc, and typically 6*9 format which isn't that useful). There is a lot of fun to be had with the fuji rangefinder range, and i'd highly recommend one of the Fuji GA645 cameras:
http://camera-wiki.org/wiki/Fujifilm_GA645
Here is some serious Nikon porn;
A cracking wildlife photography channel too.
I'm a great fan of old, uncoated lenses for all the reasons you point out.