I've used this guy
http://www.avsvideo.co.uk/
£8 per hour (of content) to convert to DV (.avi) I think is very reasonable. The price includes the DV tapes, and posting them back to you.
If you want to do it yourself, and want to convert to DV (.avi) - you need to be able to play the High 8 tapes, and you need something to do the conversion. Some digital 8 camcorders will play High 8 tapes
and convert them (output DV), so if you can get your hands on one of those that's an option.
Otherwise, assuming you get a High 8 Camcorder - if your DV camcorder has AV-input and AV/DV pass-through, you can use it to do the digital conversion. Do you know if your DV camcorder does this? What model do you have?