I would go for a second hand Boardman Comp Hybrid for around £120 on
eBay - they are very common and can be found relatively local if you are prepared to drive for a bit to pick one up. Chances are you'll find one in very reasonable condition.
In terms of safety it just needs to be able to stop. A quick once over to make sure all bolts are at the correct torque - what else could go wrong?
Regardless of its second hand cost, a couple of gold rated locks are still advisable if you are leaving it unattended. So about £100 on locks, no matter what bike you have.