You would need to get along and view some 'for sale' cars to have a true understanding of what you can get for your money. Age rarely comes into it but the engine fitted and quality of the build does. £6.5k just gets you into the Redtops and Zetec engined cars. As you're handy with the spanners you can make the car your own and improve it over time. Don't forget to ask advice on this forum, there are many tried and tested upgrades you can do.
Insurance premium can be reduced by having a limited yearly mileage, being a member of a car club, using your garage overnight, having a parts only insurance, having high excess level and of course shopping around the kit car specialised insurers.
If you go to some club meets you'll have plenty of advice and know what to look for on a Westfield. You could post up a link to any Westfield that interests you, members will soon give you the yay or nay on whether it's a good starter project for you.