i strongly reccomend songs such as fuse and step up.
If you can't buy it then don't bother... get it free off of Youtube. Legally it's a bit of a grey area, but as long as you only keep it for personal use it's legal. Just don't send it to anyone else after ripping it. If it's kept for personal use that's fine, and if what Woco21 above me said, that they already said if you can't buy it then rip it, then that's alright anyway!
I know you can get a remastered version of the Xero tape somewhere, and I know that a lot of those demos you are talking about get released through the LPU as well. You can also just record them off YouTube if you can find the right software, but the 'legality' behind that is still a grey area.
Some of these are on singles of theirs, and on LPU CD's as well. I think they actually did release a CD with all the HT EP on it. It was the first LPU CD, and had all the HT EP tracks on it, and i think the cover might have been different as well, but I'm not sure.