I'd suggest a used road bike. In that price range a bike with an aluminium frame from the 2010s should be easy to find.
If you can buy a refurbished one from your local bike shop with some warranty that'd be the safest.
If you are not comfortable with drop bars my next suggestion would be a fitness/trekking bike, without any suspension.
Size fitting is important, check out any calculator what size would work for you.
Also you have to find well fitting contact points, especially when you start to go on longer rides: a comfortable saddle (harder is better in the long run), good grips/bar tape and pedals + shoes (clipless pedals later on perhaps)
As for training I don't think you can overdo it within sensible limits, your ass will hurt before that:).
You can check out interval training for building up stamina