Bah, that's WAY the hell too much to pay for a case. They are nice cases and the AMD64 system I built I suggested the customer get one and his was $125 but had the slideable motherboard tray. For the price of that case they should have left that function in there and pisses me off ( even though I don't own a lian-li) that they'd remove that awesome function. There's no point in spending that much on a case that doesn't even have THAT great of functionality and practicleness....I personally don't see why they gave it 10 out of 10 I dun like it!
Sry to bash your case apok
Back to your question though....it really depends....would you rather 'mod it' or simply order a seperate part? Depends on what you'd rather do with your time. If you order the window you know it'll match, and if you get a kit there's a chance of scuffing the outside of your existing panel I say, but really you gotta ask yourself if you'd rather simply order it and have it delivered to you or go geek style and mod it in there yourself.