A standard 1500 without overdrive at 70 will be revving just north of 4k, depending on whether it has standard tyres/wheels, but a 1500 with O/D will be sitting at about 3.3k.
http://www.mintylamb.co.uk/gearspeed/
I currently have a 1500 O/D for sale but I'm quite far away from you and I think it's being picked up at the weekend. I can remember doing 70 in it and it felt absolutely fine however, it was on Sprint wheels with 175/70 tyres which brought the gearing down a couple hundred revs. In my opinion cruising at 70 at 3.3k should be fine, but I wouldn't want to go any faster for long sustained running. You also have the option of fitting different wheels/tyres to bring the gearing down a bit more. I think for you and any motorway work, O/D is essential, and at the price of 1500 O/D gearboxes these days (£250 - 300 for a good one) it'd be best to buy a car with it already fitted than to buy one and attempt to convert it.
As for the bottom end on 1500s, yes I've also heard that they're weak but I haven't had any problems myself. As long as you stay clear of the redline (5k limit) and the engine is in good condition with good oil pressure, clean decent oil + filter, it should be fine for normal driving.