I would like some help with buying a tap & die. Do I need a metric set or imperial set.

Both. I got a decent set of Clarke ones from Machine Mart that were 90% metric but included a few useful BSP sizes that features on Defenders.

The cheap ones really are crap, you wont realise how crap until you use a decent set.
Works buys the sealey ones, utter pants and thats being polite about it!
I just buy the set of 3 of off ebay cheap and real proper quality, Dormer Rotagrip and most british makes are the stuff to go for, tbh pretty rare I use a tap these days.

I still have some of the old Draper ratchet tap wrenches, but all my taps and dies are Dormer.

yep, taps and dies are one of those items where the price really does make a difference.
I wouldn't buy from ebay or online until you've got your head round wha'ts good and what's not, find out the local tool shop - the one all the local factories and workshops use, it'l be full a right bunch of nutcases - my local one certainly is .
But they'll help you out if your patient and only buy the sets you need, as a basic, M6,M8,M10 and M12. there will be more, buy the sets of 3 as while the singles that come in the cheap sets are good for cleaning out an existing thread they will break off when you try to cut a new thread in thick metal, and believe me, you really don't want to go there .
then buy old tap wrenchs and die stocks from car boots or local markets - they tend to be old British makes made of steel rather than the modern monkey metal junk.

last bit of advice, buy a tin of cutting paste and use it, it lubes the tap and newly cut thread.

The ratchet type T handles for the taps are one of the best things I bought many years ago.