Invest some of your pension in this book-
http://www.amazon.co.uk/Glass-fibre-Han ... 0852428200
It's an excellent book describing all the techniques you need to know for hard case GRP lay-up.
I find that PVA on it's own is a bit risky, and will hold your lay-up a bit too well. Use PVA in conjunction with a good non-silicone based wax (canuba).
You can purchase all of this when you get the rest of your materials.
If you decide to use Polyester resin for your model, bear in mind that it pongs to high heaven. This isn't a problem for me, as I have a workshop, and I quite like the smell of polyester resin (I know I'm a strange bloke). But if you're making your model in the house and you have a Missus, be prepared for some domestic friction. Epoxy resins are far less pungent, but they are very expensive to purchase in this country, and in my view the cost to performance ratio doesn't justify the price.
You can make the basic plug from anything you have to hand. A lot of people use foam because it's easy to work and light. It's also quite cheap.
However you can use timber, plaster, metal or plastics to make your plug, it only has to be strong enough to withstand the handling involved with moulding. Use what ever method you feel most comfortable with.
Regarding getting the plug perfect. Obviously the better you can get it, the better the finished mould will be, but you don't have to be too fussy, unless you are producing kits, as any blemishes can easily be sanded out or spot filled when you are building it up.
The main thing to avoid is undercuts, which will result in nasty lock-ins. Again this is explained in the book. If you mould in rubber, undercuts are less of an issue, this is one of the reasons a lot of modelmakers use rubber tooling, especially when the model exhibits a lot of fine detail like plate lines and rivets. Rubber tooling also requires virtually zero preperation, i.e. no polishing and PVA'ing.
As you're interested in a TB4, a hard case mould is ideal for a simple shape like that, and much cheaper than using rubber. You can use GRP for the mould, or you can use plaster. If you go for the latter, use a special tooling plaster. Tiranti sell a plaster called 'Pure Alpha', and it's excellent. It has a very high exotherm, and sets off in minutes. You can comfortably demould in about two hours, although I prefer to leave things overnight, just to be sure.
An extra thing I always fit on any mould, are small holes where I can insert compressed air. This makes mould release a quick and painless task. Before lay up, I fill the holes with little pieces of plasticene. This stops the gelcoat from leaking through, but easily blows out with the mould when the air is introduced.
I have a compressor to supply the air, but you can use a footpump, or even a bicycle pump. Much easier than using little wooden wedges, believe me!
You will end up with some little dimples on your work where the air holes were, I just sand them out (takes seconds).
The main thing to bear in mind is, NEVER EVER skimp on the polishing and PVA'ing on your mould before lay-up. Else it'll all end in tears. The prep is everything.
One last thing, mould something small and inconsequential first. Carve a little skiff or something, and practice the techniques on that. If you get it wrong, no big deal.