I think your biggest snag here is that you seem keen to buy new. Some people do feel the need to do that but on the other hand, I've picked up some stunning rifles on the used market so I personally feel that you're really limiting yourself for no reason other than the modern trend of second hand being a bit dirty!
When folk bought a washing machine in 1970, they expected it to run, and run, and run. There are still old folk out there now using knackered but functional 1970's machines 50 years later. How many 5 year warranty modern machines do you reckon there will be, still working, in 2070? I rest my case!
My advice - pick a cartridge (the hard part), slap in a variation, and get looking. There are 1970's lever guns out there which have barely been fired, they're considerably better than new. As you know, my .45-70 is on offer - I'm passing it on because I've now got a .375 Winchester. I don't know the age of that because I bought it based on condition and manufacture ended for that model in 1986! But none of the lever gun cartridges are what you would consider overbore. The barrels on them will last decades, centuries even with a bit of care. Don't be afraid to handle a few, even ask for them to be checked over with a bore scope if you're worried, but most will be fine. They've been racking cartridges for 50 years and will do the same for another 50 with absolutely no trouble.
£500 and a bit of leg work will see you with a better rifle than if you go and put down £1500 on a new one. I haven't bought a new rifle for years and in the 30+ guns I have bought, at least 75% have been second hand and I haven't had a bad one yet.
Oh, and if you want a light rifle for knocking over deer in a wood, don't buy a .45-70. They're massive. Get a .30-30 or .35 Remington if you can find one. By all means have a go with mine (the offer is always there), but it's a big old brute and much better suited to offering the big bore bang and recoil buzz than a quiet walk in the woods! I guess it depends what you're looking for, and that's not always an easy decision to make.