Geoff, ride-on mowers seem to need a fair amount of maintenance, like reel-type mowers and English sports cars. You have to expect to spend your weekends dispensing oil, grease, tweaks to adusters, and TLC generally. I think if you look after the bits that wore out and went rusty, those specific problems need not recur. It sounds as if the previous owner didn't look after the moving parts, and probably didn't maintain the oil level in the engine either. Sometimes non-runners can be worthwhile so long as the price you pay is in proportion to the work you have to do, and you personally enjoy fixing things and making something out of nothing.
Glad it all worked out. If you have further problems or questions, just come back and ask. Meanwhile I'll close this thread. If anyone needs to post to it, just send a PM to a moderator.
