I've just bought a second hand Type 4 2L motor, hopefully in good condition, for my Kombi to replace my tired 1800. The Kombi is currently running with a 1600 gearbox.
Do I keep 1600, upgrade to a 1800 or 2L ?
The guy I got the motor from told me that it has an 1800 flywheel, cluth and plate fitted so it will fit all box sizes, is it really that straight forward ?

I'm using the Kombi as daily transport in the city and an explorer at the weekends.
:beer

Maybe, Maybe not,
Besides the flywheel size issue,which should be fine, I believe the gearbox input shaft on a 1600 t1 box is shorter because it is designed to bear on the inside of the gland nut which holds the flywheel on.
with a t4 engine (1700,1800,2L) there is no gland nut but there is a bearing in the end of the crankshaft.
So if the shaft is too short to reach the original bearing it will not have any support at the front.
Normally what is then done is to drill the flywheel centre to take a pressed in bearing and remove the one in the crankshaft.
Your existing 1800 flywheel may or may not have had this done.
Jeff

If you have sytock wheels then you want a 2L box. It has much better ratios than the other and will help you out heaps.
The 2L with the 1600 Box and stock tyres will scream its heads off.
Goto OffroadVW.net and download the gear calculator to see all the ratios and effect they have. It is under tech section.