The Japanese manufactured Howa AR-180s are generally considered to be the nicest of the lot. The earlier manufactured (produced earlier in time than any of the other versions) American made Costa Mesa rifles are a close second (and they are rarer).
The last-produced British manufactured Sterling AR-180s are the roughest of the three, with a fair amount of variability in 'quality' over the production run. The Sterling guns are also the most common, with many more manufactured than either of the other two 'versions.'
I have a relatively early Sterling AR-180, SN 154xx, and it's one of the nicer Sterling guns that I've seen. I've been very happy with it.
In spite of my comments about UK manufacture of the AR-180, they are generally still quite good guns.
Some parts are available for the AR-180, probably coming from that old-guy-out-west's stash. I haven't looked for parts availability information in a long time. Then again, I've never needed a part. But I do like to keep spares for some things.
The original 20rd mags that came with the Sterling AR-180 rifles were covered with a very glossy black paint, and the magazines were manufactured from a very soft aluminum. They worked well, but wore rapidly.
Thermold also manufactued a 30rd synthetic magazine for the AR-180 that usually worked reasonably well.
Several manufacturers offered magazines that fit the AR-180, with differing levels of reliability. Many decades ago, I purchased ten steel 30rd magazines (parkerized) that were cut for both the AR-180 and the AR-15 (but without the bulge in the side near the AR-15 latch-cut in the magazine). These magazines have always functioned flawlessly in both guns. I have no idea who made the mags, as they are unmarked. They still work perfectly after all these years.
I also have some of the Ramline 3-in-one plastic magazines mentioned by kBob. The somewhat flimsy plastic has held up better than I originally expected. The plastic is flexible enough that the feed lips can spread apart when the magazine is fully loaded, consequently, I don't load them to their capacity most of the time. I have found the Ramline mags to be excellent performers in the Mini-14 and the AR-15. They don't work for crap in my AR-180, with constant feeding problems.
Since the production of the AR-180 was relatively small and since the AR-180 was touted as a perfect light assault weapon for survivalists (and it is) back in the 1970s, many of the AR-180s have seen quite heavy use and are in less than sterling
)) condition.
If you can find any of the three in decent condition and for a decent price, snap it up...
Good luck..
Forrest