Recording ambiences needs high quality preamps and microfones. If you want to record sensitive things like wind, grass and trees, you need an good recorder if you want some sound level on the tracks and not only the hiss from bad microfones.
Recording 5.1 ambience you need in minimum 4 channel for quad. Take 4 Neumann KM184 with windshields and stands. If you take omnidirectional mic for the rear, you get a nice and wide surround.
For recording ambience, you need to rise up gain very high. If you're too close, you hear yourself breathing on the tracks, also you cannot hold mics in hand.
Wherever you are, you need good windshields. Omnidirectional mics are less sensitive against wind but you need to protect them too, otherwise the result is unusable if you have a lot of rumbles on the tracks. Imagine, you have now 4 or 5 channels to control. If you have rumbles or noise on one channel, it affects all the other channels too. Also you need to run them in sync, another point to take a 4 channel recorder in minimum instead of 2 individual recorders.
If the budget don't allows to work with professional stuff, I honestly would recommend to record the atmospheres stereo. It is better to have well recorded stereo atmospheres with windshields and good microfones which delivers a good sound level instead of surround atmospheres which were not used because the post production of the tracks is too much work. Keep that in mind: What you have to do for one track (sync, level adjust, pan, positioning, cleaning...) you have to do for every track you record.