Sadly, the words 'economical' and 'self-build' do not go together nowadays. The bulk-purchasing discounts that most suppliers can achieve will generally mean that pre-assembled PCs will be cheaper.
There are several alternative routes - is there a good local shop that you can go to, talk to them about your exact requirements, and see what prices they will charge?
Or click here and use the 'build-your-own' option.
With any of the above options, you will save yourself a huge amount of time and potential grief. Identifying each required component, sourcing at a good price, assembling, troubleshooting etc can be a nightmare for the novice, especially if each supplier says 'it's their component that is incompatible with mine, not mine with theirs'.
None of the above is intended to deprive you of the huge amount of satisfaction that comes from assembling your own PC - but you did ask what was involved in building your own :)
Don’t forget guarantees and the cost of a new licence for Windows.
See click here for one of many home-build sites on the Web