If you’ve got a bad back & want it for longer days in the saddle then I’d start with a steel frame which you should be able to pick up for £200-£300 off of the classifieds.
Wheels again a fairly decent set of lighter wheels 2nd hand can be had for about £200.
Forks depends on what you’re after but a set of Manitou Minutes (light & good enough) are about £300 – £350 on CRC IIRC. You might get a cheap set of old Rebas for £150 – £200 if you are willing to wait.
The rest can be had easliy enough from the classifieds but I’d budget about £250 to cover is.
I guess you’re looking at between £800 – £1100 for the full build, possibly cheaper if you get lucky.
Of course once you’ve got it built up you can recoup your cash by selling the bouncy bike which will just be gathering dust in the shed 😆