I'd recommend two. A pair of clippers and an xacto knife. Clippers from GW are fine, I'd buy the knife elsewhere. Hobby Lobby or Michaels will usually have them cheaper, particularly with a 40% off coupon.
@Padre is correct, those are essential tools. I'd also recommend getting a couple of small files, as they are very useful for getting rid of mold lines. You definitely don't need to buy them from GW, a hobby or hardware store will probably be a good deal cheaper.
It really depends on several factors. Plastic glue: It is the best choice when assembling plastic models. It literally melts the two components together. Keep in mind that this type of glue will not bond metal components. Only use it when gluing plastic-to-plastic. Two part epoxy: It is the best choice when you need a very strong bond, as in the case of very heavy metal components. I find it works especially well alongside of pinning. On the negative side, it is much more troublesome, time consuming and annoying to work with. I only use this type of glue for very special types of to jobs. Superglue: Works on all models no matter what material they are made from. If you could only buy 1 kind of glue, this is the one I would recommend due to its versatility. However if you are gluing plastic models together and you have access to plastic glue, I would use plastic glue over superglue. So there is no "best" glue in all situations. They each have their own relative pros and cons. Having all 3 at your disposal is always handy, but if you wish to save money, then go with superglue if you have models made of different materials (metal, resin, plastic) or plastic glue if you are gluing together exclusively plastic models. The one you would use least often is the epoxy, you can probably get by without it. Hope that helps
I keep two types of glue around. I buy Testor plastic cement from Hobby Lobby and Krazy Glue from a lot of places. I think the last batch I bought was under $2 after tax for a two-pack at Wal-Mart. I use the plastic cement for plastic models and the Krazy glue for both metal models and GW Finecast.
Yeah for finecast you will need super glue. For plastic the plastic glue is much better. I also keep both on my table. You can use cheap nail files (those small ones that are made of cardboard and sand it seems) if you have to file something. Clippers and a small knife are a must. Don't buy them from GW, there are cheaper ones that are just as good or better. As for knives: I use scalpels (cheap ones that are meant for one-time use) actually, those are also good for removing mold lines if you just use them to scrape. But a more sturdy knife is also good sometimes, like a box cutter or so.