A few snippets which might help.
1) I get the impression you'd mainly be cycling on the road, and at worst on "good" tracks like canal towpaths and the like, rather than off-road ravines. With this in mind I would absolutely not buy any form of suspension. It's probably actively "a bad thing" even for tracks, adds weight, and uses up money which has thus been robbed from the quality of other aspects of the bike. You really don't want it for the road, nor farm tracks really. Suspension is for proper off road stuff, not merely roughish roads.
2) tyres: If doing tracks, you might want slightly wider tyres - say 30 to 35mm, though 28mm would do at a pinch. 26" wheels would be slightly better off road, and perfectly OK for the road. Go easy on having tread on them - no tread at all is best for tarmac, maybe a little bit of tread if doing muddy tracks.
4) Consider practicalities like mudguards and pannier racks - well worth having in my view, though a saddlebag or bar bag would do if you're only carrying spares and sandwiches. Panniers are better if carrying shopping, or clothes & computer to work etc. Mudguards are deemed "not cool" but an apparently explosive dysentery spattered arse is OK.
5) as a rule harlfords is not highly regarded for advice or quality or assembly, even if (say) their boardman range is regarded as perfectly OK in themselves.
Halfords do sell some really bad cheap bikes too, so it's definitely caveat emptor.
6) saddles are tricky. Nearly everyone here would advise against a "gel" saddle or any kind of padding. Some, including me, would recommend a Brooks leather saddle, but other perfectly sensible people think they're instruments of torture. It's a bit of a try it and see problem which could be costly if you don't get on with the first few.
6) I can't help with the bad knees question, and though I'd generally advise at least considering a drop bar rather than upright bike, I'd not presume to advise this if you've any issues on getting comfortable. Mostly it's people with bad backs who want upright though
