For microphones, the best bang for your buck with the added value of low self-noise...Rode mics are my "go to." I have a pair of NT55: small diaphragm condensors with exchangeable capsules (cardioid and omni) included. You can look a factory matched pair and their specs here
If you are recording ambiences it is nice to have a stereo pair and a stereo bar (or even dual stands if you aren't hiking too far out) vs an M/S pair or X/Y stereo mic in a single blimp. Using either ORTF or Spaced Pair is ideal for ambiences. Of course X/Y and M/S are fine, but the stereo image you get from ORTF or Spaced Pairs is much more defined. Granted if you are hiking out far into the wild or running all over the city, the lighter and less gear you have the better.
Also, self noise is the spec you want to check out the most in both your mics and your recorder. Schoeps mics are amazing microphones and quite expensive, but they have a fair amount of self noise and thus aren't the most ideal for quiet ambiences. People use them all the time, but Rode mics or, if you have a bit more money, Sennheiser MKH series are some of the quietest mics out there.
Finally, what recorder are you using? I don't have much experience with the Sony D50 mentioned above by Marco, but as far as I know it is a decent handheld but not ideal with external mics, supposedly it does have some of the better onboard mics for handhelds out there. Sound Device products are amazing...wonderful pres, compact, robust, the whole package, though quite expensive. I bit the bullet and bought one and have never been happier. You could look into a 702 here or a 704 and just see what you think. If it is something you are going to be doing more of, it is a great box and I think worth the investment, but obviously you know your budget and need to be comfortable with your purchases.