Then with respect, you probably need to find yourself a new hobby, because fixing these old machines has no guarantees attached to them. A lot of it is trial and error - there are no guarantees anything will work 100%.
However, being slightly, but not hugely philanthropic in nature, I'll offer you a deal.
I can see you're in the UK, like me, so what I'm prepared to do is this. I'll send you FREE of charge a known working (known working because I will have tested it) SCSI hard disk with a preloaded System 7 for your particular machine. If this fixes your problem, in other words it was simply a bad Hard Disk drive, you can keep it for free. If it doesn't fix your problem (as I suspect it won't) you can send it back to me at your own expense. I'll also have a look for some RAM simms that might work in your machine, but no guarantees on that because I haven't looked yet at what I've got or what your machine needs.
Let me know if this is something you're interested in and if you are pm me your address.