I went with an untrimmed kit. I wanted to ensure that everything fit well and wanted to experience the start to finish process. I have seen both sides of the pre made kits with some very poorly fitting when they arrived and essentially had to be rebuilt and some pretty close to right on. The trimming is not too hard. I used lexan scissors to cut the basic trim, then I used a belt sander for the bulk of the closer trimming. A Dremel or rotary tool is also very helpful for smaller detail areas. Needle files are pretty critical for the teeth on the helmet.
For polishing , I use 1000, then 2000 grit wet sanding blocks if there are scratches, then follow with the Novus number 2 polish from Trooperbay, then Meguires Plastix for the final. For light scuffs, the Novus alone works well.