Well, as moderns go, I do like 75s a lot. I prefer the P6, but I've done a serious number of miles in 75s and ZTs.
The 75 I've done most miles in is my 2.0 V6 Club SE, which I run on LPG. I've found that 75s are pretty reliable things, for the most part, but, they're not the cheapest cars in the world to repair. V6s are my preference as I've no interest in running the underpowered 1.8, the chronically unreliable 1.8T, and I don't like diesels. If you're looking at 75s, the V6 is available in 2.0, or 2.5 form, with manual or automatic gearboxes available in both. The 2.0 auto is pretty lethargic, based on trying a couple. The 2.0 manual isn't too bad, but somewhat lacking in torque, meaning that you'll find yourself changing gear more often than perhaps you'd like. 2.5 manuals are pretty quick, but I reckon the 2.5 auto would be my choice, in an ideal world, having owned a 2.5 Connoisseur SE Auto, and a ZT180+ Automatic.
The V6 is a pretty reliable engine, for the most part, and my 2.0 has done 40,000 miles in the two and a half years since I bought it first. Cambelts are a bit of a pain on the V6, with there being three of them, and you want the water pump changed at the same time. Budget around £500 for this work if it hasn't been done. Thermostat housings can, and do, crack so look for coolant in the vee of the engine, and watch for coolant loss. The cooling fan needs to be working, but often fails. It can be checked easily, if you turn the car on, and switch on the demist setting on the climate control. If the fan doesn't kick-in at this point, it will often just need a new resistor, for around £15, but, in some cases, the entire fan can need replaced. If there's a rattling noise while the V6 is running, it'll often be the inlet manifold which is the source. The V6s have a VTEC-style system, where there's quite a kick at around 3000 rpm. This is controlled by a pair of VIS motors, which do fail, but can be refurbished at modest cost. You'll notice a big improvement if they're working properly!
In terms of the rest of the car, watch for knocks from the suspension at the front, which can be caused by drop links, wishbones, etc. Nothing outstandingly expensive, but, at the age the cars are at, budget for wear and tear items. Rear springs can crack, and you can only definitively check by getting the car up in the air. Generally, the car will sit a bit low if they are cracked, at the rear. The rear suspension arms can rot pretty alarmingly in some examples too, so worth getting underneath prospective purchases. Exhaust systems will need replaced on a lot of examples. Cheap exhausts are very boomy, and unpleasant. Fit an OEM one, or something from the likes of Bosal, and you'll be fine. I've found 75s aren't very sore on tyres at all, and I drive mine fairly hard. Handbrakes will often be useless, but this is easily sorted by replacing the compensator.
Other things to check, are for a swimming pool in the spare wheel well (the majority will have been like this at some point or other), easy to solve by sorting the rear light seals, in the majority of cases. Watch for damp carpets too, particularly in the front. These are usually caused by blocked plenums, which are in front of the bulkhead, and the drains need to be kept clear. Easy to do with a drain cleaning tool, to drop down the drain every couple of months.
Bodily, they don't rust much. I'd be inclined to avoid Cowley-built cars (black sills, and black trim on the bottom of front and rear bumpers), as I've viewed quite a few which have the beginnings of sill rot, and they just don't seem half as well rust-proofed as Longbridge built ones. Front mud flaps can cause rust on the front wings, and I've seen quite a few with rust on the leading edge of the bonnet, rotting from the inside out. Any other rust can only be attributed to very poor accident damage. I've seen quite a few Tourers with rust on the rear wings, where I reckon they've had accident damage.
Interiors wear well, but the plastic trims above the kick plates can fall off regularly unless you replace the clips. Just watch for the airbag light. It's common for it to appear, and more often than not, it's a poor connection under one of the front seats.
In terms of models, you have a bewildering range of choices; Classic, Classic SE, Club, Club SE, Connoisseur, Connoisseur SE in mark 1s. (I won't bother with mark 2s, as I assume you're after a mark 1)
As such, I would go for a Club SE, or better, spec-wise. Club SE has climate control, 16 inch 'Union' alloy wheels, four electric windows, Velour upholstery, fog lights, etc.
Connoisseur gives you electric leather, and heated seats, as well as chrome mirrors.
Connoisseur SE has a lot of toys, including a better radio, electric rear sun blind (always amused passengers 8) ), cruise control, memory on the driver's electric seat, etc.
Options that people like include electric folding mirrors (pointless to my mind), larger wheels (Connoisseurs and Connoisseur SEs only have 15" wheels as standard), sat nav (useless, to my mind, and more often than not, the module is damaged by water ingress into the boot), walnut steering wheels, and any additional chrome are selling points to many.
I would avoid buying a Cowley car, personally, as I think the rust-proofing is very poor, compared to Longbridge ones, though they are marginally quieter, and are all pre-project drive. An early Longbridge one would be my first choice, ideally a 2.5 Auto Connoisseur SE.
Here are photos of the two 75s, and the ZT I have owned. I'm a regional secretary for the Rover 75 Club, so if you want to know anything further, I'll do my best to help.
My daily 75, which covers a lot of miles, often in a rush, and never complains.
ZT; Now sold. Bought for £270 with a water leak, which cost £20 to sort. Covered 5000 miles in it over the summer, and it caused no issues. Sold to my mother.
Very early T-reg 75 Connoisseur SE, bought for £350, with almost a year's tax. Had the highest spec I've ever seen, and amazing history. Unfortunately, the chap I sold it to decided he'd break it. :roll: