Maybe Queensland or Northern Territory, Australia.

" ......lifestyle is quite cheap, with tons of mango trees and street markets...."

India would be your best choice, they have very high quality mangos, and it would meet your other criteria as well. In the tropics mangos are a winter/spring crop, in the colder areas they're a summer crop. And of course the best quality is in the winter/spring fruit. In Australia you'll get the best quality as well, but certainly not a cheap country to live in, especially the Northern Territory.

Limpopo province in South Africa. Really beautiful sub-tropical climate and as a bonus you can go on safari. Driving through the province you'll find lots of places to buy all sorts of fresh fruit. If you're using pounds you're going to find the exchange rate and cost of things extremely favourable.

India does stand out. I myself would choose the country of Hawaii. Natives are friendly and speak English.

This post could use a few smileys.

not to hijack the thread
but, i am looking for the same thing, but closer to home
(Louisiana)
i was thinking of a VACA in the Caribbean, Costa Rica, or similar.
Trying not to have to travel too far.
i like South America, but, its a long flight for me.
I wanted to include tropical fruit tasting, and maybe picking up a few seeds

If you're looking for a large range of tropical fruit and not fussy about absolutely top quality then it's not that difficult. Even tropical fruits come from a diverse range of climatic conditions. Where one species will grow really well another may be of very poor quality. So virtually anywhere in the Caribbean should give you a good range of different fruits. They've been collecting and growing them for centuries.

The Fairchild's Botanical Garden has a Mango festival every year. Check out the link. They have Mangos from all over the world. (I only have one Mango tree) It's Miami so no small town. You could take a ride to the farms south of Miami, Redlands area. Many tropical growers there and a small town feel. I've never been out of the US so can't say about other areas.

Here is a link that might be useful: Fairchild Mango Fest

South FL has a lot of mangoes for sale between middle of June till late August...Jamaica, Caribbean are a couple of places I've visited that has a lot of mangoes in the summer as well...