The VRT is greatly determined by year and fuel type. You'll pay considerably less VRT if you go for a diesel. I recently purchased a car from NI and saved several thousands based on listed used prices on websites here.
I'd advise you to avoid private sales and stick with dealers. Look for cars that've passed a used vehicle check, such as Network Q. Also, you could pay for a vehicle inspection by the RAC, AA etc., although this is a little pricey. Preferably, take someone with you who knows a bit about cars. It may also be worthwhile checking the car is 100% legal by using an online service such as cartell.ie. It would be highly unlikely a dealer would sell a stolen car!
If you import a car that is 4+ years old you must arrange an immediate NCT. It's less hassle to buy a younger car especially as you'll be sorting out insurance, VRT then motor tax.
Finally, I'm not certain as to how willing dealers might be to accept a trade-in. Factor this into your costings.