Having TimeMachine backups makes a big difference in your options.
While I think that your internal HDD is having problems, it's possible that just the OS files were botched and that your data is fine. It's also possible that you won't be able to restore from your TimeMachine backup. So a prudent action to take is to get an empty external HDD that is at least the capacity of your internal HDD and connect it to your iMac. Boot from your Snow Leopard disk and one option should be "Restore From Time Machine Backup". Instead of selecting your internal HDD as the target, select the external HDD and see if can restore to it. This process will like take a while to complete.
If you need to buy an external HDD, you likely will have to format it for the Mac using Disk Utility which is an option when you boot from the Snow Leopard install disk. The Disk Utility app had a major makeover several OS versions ago so the following is a link of how it appeared in Snow Leopard. It was the first link I could find that had pictures - there's probably better examples but this will do. In the link, they start Disk Utility from a working computer - you're starting it from the install disk. You also have to be aware that this link was made for a specific product and to ignore references to that product. IMPORTANT. In step #4, select "GUID Partition Table" instead of "Apple Partition Map".
https://datalocker.com/how-to-partition-and-format-the-datalocker-on-mac-osx-105-or-106/
To summarize on how to format a new external HDD:
1. Open Disk Utility from the Snow Leopard install disk. At some point, it may say the drive is unrecognizable or something similar. You can ignore this message.
2. Select the empty HDD - make sure to select whatever is to the left-most side that represents your external HDD. Make sure not to select your internal HDD or your Time Machine HDD.
3. Select the "Partition" tab at the top and make sure "Volume Scheme" is set to 1 partition.
4. Press on the "Options" button and select "GUID Partition Table".
5. Set the format to "Mac OS Extended (Journaled)".
6. You can name the disk if you wish. Press the Apply button.
If you don't want to go through this step of trying the restore first on another disk, that would be your choice but again, it's possible that your internal HDD contains a good copy of your data (even if it can't boot) and that the TimeMachine restore will overwrite this data with nonsense.
So if you do go through the step of trying out the restore and it works and the data is good, you can try to restore to your internal HDD.
- If the TimeMachine backup is of what you had in El Capitan, you'll get El Capitan on your internal HDD.
- If the TimeMachine backup is of what you had in Snow Leopard, you'll get Snow Leopard on your internal HDD.
If your TimeMachine backup is El Capitan and you want to revert to Snow Leopard, you should restore to the external HDD, format your internal HDD, try the Snow Leopard install again and then you'd have to copy over your data. You might be able to use the Migration Assistant app but I'm not sure if that would work.
So the prices in the UK for a SSD is £64 for near-top-of-the-line 500GB, £52 for a name-brand not-quite-as-good 480GB, £30 for name-brand not-that-fast 240GB. I don't want to hazard a guess as to what service costs would be in the UK.
UPDATE: If your TimeMachine backup is not up-to-date, after you boot from the external disk (presuming the restore works), you can check your internal HDD to see if your data files are OK (It would probably be under /Users/[your login name]. You can copy whatever files at this point - but don't overwrite files on your external HDD - copy files from your internal HDD to some other directory on your external HDD in case the files on the internal HDD have been corrupted.