Gundam kits are generally split into 4 categories - First Grade, High Grade, Master Grade and Perfect Grade with FG being the easiest to put together (and least detailed) to PG being the hardest and most detailed. FG kits are generally 1/144 scale, and HGs are split between 1/144 and 1/100 scale. MG and PG are almost always 1/100 scale. There are also 1/60 kits although I can't remember what grade these fall into. I suspect they are MG simply because their size means they would simply be overly difficult to replicate the detail apparent in a 1/100 scale kit without being obscenely expensive (and PG are pricey enough as they are). I personally find the HG 1/144 scale is a good compromise between cost and detail but I do buy HG 1/100 and MG 1/100 for the better Gundams. But the key reason I buy 1/144 HG is because it is the scale Bandai uses for the majority of their kits. Some of the less popular designs are not released in 1/100 scale which if you are aiming for consistency, really throws a spanner in the works. Plus, you will never find some of the larger items in 1/100 since it would make them disgustingly expensive (the Meteor Unit I recently completed is an example). As for skill level, I think anyone can build a Gundam, it just takes time and patience. I can do a MG kit in 4 to 6 hours uninterrupted but then again, I have modelling tools to help me so for a noob, you might find it takes slightly longer.