It's not going to be worth that much, it doesnt have a good accelerator (almost certainly an A3640), it doesn't have a gfx card, sound card, network card, RAM card, USB card, PCI bridge card etc.
IDE DVD/CDRW combo is worthless, TV is worthless, the toaster is worthless to most Amiga users (should sell separately to enthusiast wanting a toaster to get best price)
Buddah card, while ok is not a great card. It's not in the same league as a toaster flyer, FastATA MKIV, Fastlane or A4091.
Homebrew conversions to PC-Towers 99% of the time LOWER the value of an Amiga, especially A4000D's. But as you are including the old case it wont in this situation.
Do the Zorro slots line up with the slots on the rear of the tower? A great conversion could add £150+ Get some photo's posted to an image hosting website and the links in this thread and lets see how good (or bad) it is.
Not including the toaster, I'd say about £350 including the spare motherboard and old case if the spare motherboard works but is dodgy. A bit more if it is fully working and the PC-Tower is a reasonable conversion. A bit less if it's dead or missing chips especially if you dont know why it's dead.