i am really a noob at this, first thing i must sy is do not commit loads of money to your first few projects unless you are willing to make a loss. what i am doing is making an assault rifle with a limit of £5, thats a very small budjet but from that i will get the experience i need. if it does fail i have not lost a lot.
first thing i would do, dont use those fold out designs unless you are completely unsure of how to build, to fold those things you need to be patient and pay a lot of attention. i started out by going on halo 3 and saving a pic of an assault rifle and then going to bungie.net and printing the pic off but at a much larger scale so that it was about 30" plus. then use cardboard to cut out the shape and do this a few times to get the thickness you want. then try modeling a frame using the rest of the card board, then gets some foam board and build up the main structure, and sooner or later you will have a good looking gun.
or you could start in a different direction, how about armor, the chiefs gloves are easy to make, again using cardboard and foam board, with the foam board you can cut out the circles and them file down the excess. attach it to a glove, black rubber would be best for the effect, and bit of paint, a bit of varnish and your done. remember if you make a mould if you break it you can make a new one, latex takes a long time to make but is much cheaper than using silicon which is considerable more expensive.
as i said, for your first few goes, try and be as cheap as possible, you never know, if you make a good job of it some people offer to buy pieces on this site. if you want loads of easy starting ideas ask me and i will be happy to help, i can also offer you a latex mould for an assault rifle i am currently making, let me know