IMO, the "upgrade it later" argument is something of a red herring.
New mother board and processor, new memory (because your old won't work in the new mother board), new large hard disk, etc, and what you will have done is to jack up the old case and replace all its innards. The case and power supply might cost 50 quid.
I have assembled my own DOS/Windows machines since the 486. I wouldn't do it at today's price of already-assembled systems.
SC