I recently retired and decided to do exactly the same.
I can't really give you advice on the specific components but I would certainly suggest that you get a motherboard bundle, which will come complete with the RAM and processor already installed. Try looking at Novatechs site for these bundles.
It isn't particularly cost effective to buy every single component but what I did was to re-use as much as possible from the old PC such as case and some of the drives. Even the PSU if its man enough.
Do have a go. Its not difficult and gives you a great deal of satisfaction. All you need is a cross head screw driver, guts and loads of Gin & Tonic!