Another vote for a nice 90s MTB, but not one of the brands which are still currently held in high esteem - I'd avoid a Specialized, Marin, Orange and so on, and get a Raleigh or - my unbreakable tool of choice - a Muddy Fox - these have the extra attraction of having had the brand utterly devalued by Universal Cycles and Mike Ashley since they went bankrupt in the mid 90s. I'd suggest one no later than about 1991. If you get a mid 80s one it will most likely have rack mounts on the forks and at the back, but mine has two lots of eyelets on the bottom of the forks and at the rear wheel as do most of the later ones - you just need P clips or - my choice - a Carradice Bagman support collar to mount the tops. Ideally mine would have nicer wheels as the originals are a bit of-road biased, but they are certainly tough. I'm considering getting drum brakes for it (Sturmey Archer do a rear drum that takes an 8 speed cassette) for total maintenance free braking whatever the weather. This would have the added benefit of allowing me to choose from the lighter 'disc specific' range of modern rims.