you're going to have to check the sticker really, i think the SE is going to be either 110, 115 or 130 depending on the year.
being a 6 speeds means its either the 115 or 130 PD engine
110K is nothing really for a TDI lump as long as its been services when it should with the right oil (important for the PD's) then it should be fine.
one thing to consider is that on the fixed interval the 120K is a major service and its going to include a cam belt and air filter.
Rhyso should be able to give you a rough idea of price
in terms of things to check just the usual things which can be found here,
http://www.golfgtiforum.co.uk/index.php?topic=38557.0