My advice is to first think about the GAME you'd like to make.
Then check if you can basically build it with what you've got so far - and don't forget to count in stock! (because otherwise the 18 MPs you bought would have been a total waste) and BUILD THE FIRST LEVEL --->This enables you to check if your game can be built at all with FPSC. No need to buy a dozen MPs more just to realize that half of the scripts you'need aren't possible with the engine
After you've done that, check if anything is missing that you ABSOLUTELY need for this beta level(enemies, player weapons and segments, forget about the eye candy for now)and buy either model packs or single models from the store to fill the gaps.
After you would be ready to release a demo think about eye candy
In case you find my grammar and spelling weird ---> native German speaker ^^
