If I remember correctly, they were specially imported to Canada in the early 90's by Century Arms. These SL7's had 19" polygonal rifled barrels, so are non-restricted.
I was fortunate (or unfortunate!) to come into a couple of them early in my firearms collecting life, well before I realized how special they were, and they've since found new homes. However, I still have an SL6, which is the 223 variant of the SL7.
As mentioned, buffers do deteriorate, but I've been able to purchase replacements and at the time I was looking for info, there was a fellow in the US who had produced new buffers. Some of the other parts are interchangeable with other HK models (e.g. the rollers), but there are also some parts that are very specific to the SL7, so good luck finding them if you break something.
They're a beautifully built gun, and I was always fond of the side sling swivel arrangement, but I'm not much of a collector and couldn't imagine hunting the hard given their scarcity.
If you're so inclined, I still have my galleries of the two SL7's that I sold -- unfortunately Photobucket is terrible with ads, so be warned:
h ttp://s292.photobucket.com/user/jsmithsmithj/library/HKSL7?sort=3&page=1
h ttp://s292.photobucket.com/user/jsmithsmithj/library/HKSL7_2?sort=3&page=1
h ttp://s292.photobucket.com/user/jsmithsmithj/library/HKSL7_3?sort=3&page=1
And a few pictures that I re-uploaded to Imgur to avoid Photobucket:
