Never owned a Vauxhall and only ever driven a Vauxhall. They are popular with fleet managers as they are cheap to buy and run, so there are lots of high mile ex company Vauxhalls knocking about. Sounds cheap, how many miles has it done?
I tend to buy cars at the budget end of the market (500-1000) pounds and it doesn't pay to focus on the make of car at the expense of the condition of the car you are looking at. For example, I own a Toyota Corolla, renown for its reliability and durability, but there are plenty of Corollas out there with sky high mileage (they were a popular choice with minicabbers) have sky high mileage and have received next to no maintenance and will cause a world of trouble. Equally there might be a 206 that has been looked after extremely carefully and have a full service history and given trouble free motoring. I was lucky to get a decent mileage, well maintained Corolla for a good price
I would identify the size of car you want and your budget:
then prioritise
Mileage (a very low mileage isn't always a good thing)
Service History
Cambelt changed (circa 50,000 or more)
Long MOT
then Manufacturer. (There are some makes and models with known faults that can be costly so would make an older/cheaper car uneconomical to repair) but at this end of the market it doesn't do to prioritise brand at the expense of condition.
Go and visit a few cars, test drive them, size up the owner/seller.
Out of interest where are you finding the Mentor/Astra and 206 you've mentioned here?