Tricky one, 2nd hand ones are cheap enough so it's down to how much you trust any given seller.
Rock Mountain make a Roverdrive for series which is supposed to be better than the old fairey ones, and the fairey ones are still in production, but for the price of either of them brand new you could buy an LT77/R380 and LT230 and 2WD kit and have 5-speeds, much stronger transmission and a choice of transfer box ratios, plus a lower low box.
Fitting would be a bit more of a faff but if I had the best part of £1k to spend I wouldn't be buggering about with Series bits.