There must be lots on here far more knowledgeable about computers than me so I might as well pick the "collective's" brains

Basically, I rebuilt one computer about 18 months back and it's been nothing bit a PITA since

.............. OK I realise that's one of the main functions of a 'puter

but ..................... for a new machine it's always run at a snail's pace ........... to be fair I think the biggest mistake was using a Celeron chip

............. I've used AMD's for a good few years and always been happy, local computer store I always use recommended this one so that's why ........... anyway ......... enough said the Celeron chip is going on holiday ..... in the wheelie bin
but, just to avoid another mistake can anyone offer a bit of advice.
The hardest work the machine will do is a bit of 2D AutoCad and large spreadsheets, no games .......ever
I was looking at these 2,
Clicky 1Clicky 2the obvious difference is one comes in a case ......... even I spotted that one

but I suspect the slightly dearer one will be a lot better ................ but if anyone can advise me I'd appreciate it
Ta chaps

CC