RD28 Power issues

Hey all, brand new to the site. Just wanted to raise some opinions quickly. I have a 1998 GU With the RD28 in it. It's been sitting around a few months now since I last used it, when I last did, it seemed more powerless (than RD28s already are, it's not chipped). When I started it today, I noticed several things. It was extremely hard to start (but has been going on for ages, so already something going on in the glow circuit I suppose), It was absolutely spewing out out thick clouds of black smoke, seemed to have an occasional small misfire as the revs got higher, and I couldn't seem to hear the turbo spooling up. I haven't tried driving it yet, but I just wanted to brainstorm a bit before trying to do anything else to it. I have already changed and primed the fuel filter, plus dumped some diesel conditioner in the tank (diesel still seems alright at this point). It does have a fairly substantial rocker cover leak (oil all over gearbox), plus there seems to be a small oil leak from the rear driver's corner of the head. Anybody ever encountered similar symptoms? Any suggestions would be greatly appreciated.

Thanks heaps,

Tom.