I wouldn't say that the 3VZ/1MZ engines are fun to work on, they can actually be quite difficult at times due to space in their applications.
They are generally pretty reliable and do sound quite good with a decent cat back exhaust. Extractors aren't really available off the shelf(the ones that are don't fit the best) and custom ones can set you back a fair chunk of cash.
It is all depedant on the conditions the car has been driven in and the driving style of the previous owner.
The Kluger is also a fairly heavy vehicle, so the load on the bushes will be considerably higher.
Just be aware that all of the carbon built up needs to go somewhere. Do your best to get as much out as possible by turning the engine over using the starter before putting the new glow plugs in.
Did this only happened after the filter was done?
If it is the o-rings, it will get fuel up eventually, but once left for long periods, it will drop pressure/drain back to the tank.