I did warhammer before so I've got some experience I guess. 
then you'll find Gunpla building very easy like I did, you don't even need to glue (some parts might be a bit loose though so you might want to do it sometimes)
I'd get an HG, because HG is super cheap (1000-1800 yen before discount
usually, depending on the amount of plastic (so, how big it is or if it has a tons of accesories), and made for beginners. The quality is actually not bad either unless you want older stuff. There's also sooo many more HG kits to choose from than MG and RG (especially RG, there's not even 20 of them)
For the actual kit, I'd recommend the 00 Gundam + 00 Raiser + GN Sword III, because it's got good articulation for making poses, is super cheap, and comes with so many accessories, that have a lot of gimmicks like turning from swords into guns, or combining. Its backpack ("wing things") can even transform into a ship.
http://dalong.net/review/00/0h54/0h54_p.htmThere's also a RG version, which is a bit more expensive, and a bit harder to put together (because more and smaller parts) but it's also much more detailed, and has an 'inner frame'. It also came out this year so it should be easier to find.
http://dalong.net/review/rg/rg18/rg18_p.htmBut really, just get what you think looks cool
