Where the white smoke possibly comes from is said already. But since all of this engines have a soule and their own specific little "faults", I thought I tell you a bit ab out the 4.108. The Perkins 4.108 tends to smoke a bit any way, no matter new or 5000 h what so ever. The smoke increases with the humidity. Fogy days, damp nights, it smokes, nothing to worry. If the engine is still smoking more than a bit on a dry hot summer day check the fuel lines first bevor you think of a major defect. A 4.108 gives you smoking signals on the smalest fuel leak, also the lift pump can be the leaking bitch.
How ever, this are easy things to check bevor you rip the engine a part.
I had one of the 4.108''s for ten years. It never let me down and I got used to a bit a white smoke and also a bit more on damp and humit days. I did a general overhaul on it after 5000 h. The engine was running a bit softer after, using less fuel and oil again but was still smoking white a bit.
Make shure your fuel lines are ok and no aer is goin in the system cause that''s someting this engine doesn''t like at all.
Still think of the advise having a disel mechanic to look at the engine bevor you buy.