I currently have a 74 88" Series III - it has a galvanized frame, but lots of dings/dents and rusty footwells in the bulkhead. This was an older restoration - so it has had quite a bit of maintenance done to it - including 3.54 Range Rover diffs to help it get up to speed on the highway. Parts have been extremely easy to come by (including the brand new set Good Year P40's with Wolf wheels I picked up this Saturday for $700 [selling the rims for $400] - rubber made in South Africa, wheels are new Defender take offs, a Defender 90 tailgate take off for $84, new galv. windshield with glass for $275, etc. etc.) - but several of the parts suppliers have closed shop this year - so expect repro/OEM sources to start to go up in price. The last few years of pound/euro versus dollar prices have hurt.
My girlfriend loves it (more than my 40 on 39's) - and we drive it daily around town and what not, with a soft top (galvanized bows and canvas that I picked up on
eBay for $400 - po'd as I have had two Paki tops and couldn't afford to keep them). Perfect for my Rhodesian Ridgeback (funny thing being - I fell in love with Rovers from the many trips I did to Costa - never thought about it from the Mutual of Omaha perspective).
Offroad - have seen several in the Florida club wheel - including a high school buddy who recently did a Toyota axle SOA on his big merc 4 cylinder 88" (he gets on it in the mud), but I wouldn't do much in mine. 10 spline axles - weaker birfs - small and light and would wheel well - but just not stout and built up like a 40.
Reliability - again mine looks beat to death, but has a newer engine, frame, and other maintenance items. In two years - has leaked gallons of oil, but has never broke down - and the biggest repair was last month when the MC went out. Just needed a rebuild, but I picked up an aftermarket one for $115 shipped (will go back Lucas when I find a deal).
I also bought a fully restored 67 Series IIa with the lights in the grille, true tropical top (versus the sunsheet only on my 74), and lots of other goodies like an overdrive - brand new Turner engine, etc. etc. But it was too nice, didn't feel right or as tight as my 74, so sold it ASAP.
While I've always wanted a Series Rover - and they're really fun, looking back at all the money I wasted - I missed a really good deal on a true unmolested 78 FJ40 - and in hindsight - would have been better off with that.
Summary - don't do it. Get a late 70's restored FJ40 with a Paki top for Summer - and you'll be way happier, safer, and able to wheel harder when you want to.