The best thing you can do first is treat it to some MILLERS diesel engine/fuel system treatment, thus stuff really does work wonders, then start using bio, the smoke can be caused by many things, dirty air filter is usual.
The belt/s if needing doing are a doddle, there are 2, 1 is the main cam belt, the 2nd is the injector pump belt, ALWAYS change both, have seen many where only cam belt was changed, but when pump belt breaks it has nowhere to go apart from getting tangled in the cam belt and causing lots of £££'s damage, they can be changed and the cam and pump timing done using nothing more than drill bits as dowels to locate the positions.
How is the heater ?, should be extremely good, if not chances are the stat needs doing, get the hottest running stat you can get in there.
Very important you thrash the arse of it every once in a while, what colour is the smoke ?
tabetha