Please tell us the source of the statement;
" ....DVD - and DVD +. apparently, only one of these are suitable for recording from VHS tapes, but we don't know which one"
GRC - For the record, I think this is nonsense. Any DVD recorder with an AV input (SCART or Phono) will happily record through that input, whatever you feed into it; VCR, camera, laserdisc player, another DVD player (AS LONG AS there's no copy-protection on whatever media you're playing back).
I'm using two Panasonic DMR-E55's, which I got for less than £200 each. I can happily record between the two of them; from DVD-RAM to DVD-R, and vice versa, and from VHS to DVD-RAM or DVD-R, again, without problems.
It'll quite happily use DVD-RAM discs with or without the cartridge housings as well.
If your father is playing back the archived DVDs on the same recorder that he recorded them on, any issues of compatibility between DVD-R and DVD+R can be ignored as long as you get the right kind of discs for the recorder in the first place.
If you want to know more about using the Panasonic to archive VHS, post it here, and I'll fill in more detail.
Regards, Graham