If you don't mind buying a used printer on eBay I'd suggest a Lexmark "workgroup" grade laser. Very reliable and designed to print a serious number of pages. The downside is they aren't small. The duplexer is an optional extra but is often included so look for it.
I've had a T522 for a number of years (plus several bought for friends and family) and have printed 60,000 pages - it had already printed 100,000 and is still going strong. They're designed to print 250,000 between services. The cartridges do 20,000 pages and can be had very cheaply if you're lucky - there are some going on UK eBay right now £19.95 +£6.95
Newer models are T640, T642, T644 and T646.
I'd suggest looking for one that has done 100,000 pages or less.