Howdoo and welcome to OOF.
Fisrt things first, the cambelt and tensioners must be changed at max 4 year 40K mile intervals, without exception. Unless documented proof of being done, get it done asap. Good guide and DVD on here and from user Robin Hood.
Mayo on filler cap is normal when car is only used for short journeys as yours is. Head gasket failure is rare but not unheard of. Clean the cap end take it for a decent run. Chances are it wont reappear unless short journey commuting...
Smell of hot engine is probably leaking cam cover gaskets. Not the hardest of jobs and, again, a howtoo is available on here.
As to gearbox, an autobox type will advise better than I, but as long as it goes through all gears without clunking etc and does what it should, its probable that alls well.
take time to peruse the FAQ and Tech Help sections - loads of good stuff there and reassure you.
Also, post up individual probs and no doubt someone will advise....