I don't know about individual bikes at certain budgets, but the type of cycling you do is the same as we do. We have cyclocross bikes, which we absolutely love. We cycle mainly on roads, where we do well compared to mountain bikes or hybrid, but we do some easy offroad (canal paths, farm tracks etc, but no rock!). My husband and I are both 50ish, missing the odd ACL, and one of us is a scaredy cat about downhill stuff.
Cyclocross was apparently started about 100 years ago in France for road cyclists to keep fit over the winter. They used to mainly cycle on road, but sometimes wanted to take a short cut cross country.
The bikes look like road bikes, but the forks are wider (to stop mud jamming up the wheels), and to allow you to use different size tyres. They are light, so you can hop off and fling the bike over your shoulder, or a gate, or whatever. There are rules nowadays defining what makes a cyclocross bike. Those rules were changed a couple of years ago, since we got ours, meaning that you can now have disc brakes. This is good, as our 'normal' brakes do get worn away by gritty mud - which also damages the wheel.
On price, cyclocross bikes are generally cheaper than the stupid amounts some people spend on both road or mountain bikes. About 4 years ago, our bikes should have cost £1000 each, but we got them half price from a local triathlon bike shop, as they were end of line (last season's colours, presumably....)
We do the Scottish Coast to Coast each September, training hard all summer, and I also used to cycle to work along country lanes and cutting through woods and footpaths occasionally. We do get the bikes serviced a couple of times a year and have had to replace brakes, handlebar tape and wheel rims, but basically they still look brand new.
I don't know what the secondhand market for cyclocross bikes is, but I recommend you investigate it, and your local independent bike shops to see if you can get a bargain like we did.
