It really depends on how low your budget actually is before anyone could give any advice on what to buy.
Based on the info you have given, I am going to assume you already have a HDD, PSU, case, KB/mouse & a monitor? Those things alone can set you back between £250 - £300.
Also, to save some money you could just go for 1x8GB ram stick? I am running 16GB on my main PC and 8GB on my secondary one and I haven't seen any real difference as far as memory goes.
If you are really restricted for money then it may be worthwhile looking at Used PCs for sale. I picked up an i7 4770 for £130 and a GTX 970 Strix edition for £90 both for my secondary machine which saved me a bit of cash.