The touchscreens on the xenarc lilliput are exactly the same as some of the addon ones you can buy so the only difference between what you do and what you buy already done is how well you mount it, but it shouldn't be hard to match the mounting of what you get already done. Depending on the screen, it might be hard to fit it in the original case since it wasn't made for it, but if your not using the case, then I wouldn't see any drawbacks to adding your own.
The touchscreen will take away from the brightness dramatically expect to lose about 150 nits. This is regardless of whether you get it with the screen or do it yourself.
Where are you going to find a better screen then the xenarc or the lilliput? If you can do better I would recommend going that route.