Spent a lot of time when I was looking for a forklift truck as I was kitting out my shed with pallet racking to maximise space. I learnt a lot about secondhand forklifts so here's a list to help you out.
1. Most secondhand forklifts have had virtually no maintenance their entire lives. Expect oil like treacle... or milk, as a head, head gasket or welsh plug has let go. Reason why water in the oil? No-one bothers with proper coolant in a forklift and the engine corrodes away from the inside.
2. Get a container mast forklift. This means the mast is low enough to go in a container which means it will go into most sheds and be much more useful. Sections will extend plenty high when needed.
3. Get sideshift on the fork carriage as it makes it so much easier to work in tight places.
4. LPG is the best fuel. Why? See point 1 above. LPG systems are very simple and low maintenance and can cope with lack of care. When the fork is used for short periods and doesn't get the motor very hot, LPG powered motors don't suffer. Use a diesel fork in these conditions and the motor will suffer as diesels work best and cleanest when motor is running under load at operating temperature. Petrol motors are OK, and many LPG forks can run on both petrol and LPG. However when the petrol fork sits for long periods between use gaskets in the fuel system/carby dry and leak air or fuel. LPG can be cheaper to buy as well.
I found having a forklift is really useful even when you don't use it a lot. If you make sure where you run it has a well compacted surface you can take it wherever you want. Mine has solid tyres all round with a tread pattern on the front and it gets around on the dirt ok at my place.
Carrying a bit of a load will help even more as most of the weight is over the back wheels when unladen.
Make sure you don't get one with tiny wheels with no tread which need a very smooth hard surface.
Put all your crap on pallets and stack 'em or move them around easily whenever its in the way.
They make a great mini crane especially with an extension on the forks.
The manoeuvrability is fantastic and makes tight areas no problem.
They are very heavy, even the small ones, so I would be very careful loading and trailering one home.
Do your homework, inspect every machine very thoroughly and go get a good one

You'll love it!
AusDave