Agree with most of what has been written. What are your projects? Software or hardware? Hobby or study/profession?
Pis will throttle themselves to avoid overheating. But if your projects use the processor or graphics hard, you may need a heatsink or fan to keep the performance up. (I have posted
my experience with a Pi4B running 24/7 with 4 CPU cores fully loaded.) It seems different individual Pi have different thermal responses. Given your high ambient temperature, your Pi may well benefit from a heatsink or case with fan, but it all adds cost.
Pi0 are good for lowest cost, single core so not brilliant performance and 32-bit only. Get the Pi0W to add WiFi, or the Pi0WH for a fitted GPIO header if you want to connect hardware without soldering. Less need for heatsink because of low power.
Avoid the Pi(1) series, usually only second hand and less capable than Pi0.
Pi1(+) are good for some niche uses, notably with the 7" touchscreen which the Pi0 can't use.
Pi2B is only good if you need a Pi
WITHOUT WiFi and Bluetooth. Latest Pi2B and newer models are 64-bit capable.
Pi3B is superseded by Pi3B+, but if you are in that territory go straight to a Pi4B. The Pi3A+ is good if the single USB and no wired Ethernet suit your use.
Generally the Pi4B is the best all-rounder. 2GB memory is enough, if you don't want heavy memory apps, but note the web browsing benefits from more memory. 4GB is reasonable, 8GB is top of the range if you do serious software development but a luxury for most of us.
TLDR: Pi0 (probably Pi0WH) or Pi4B 2/4GB -- read some more and decide what you need to last until you want an upgrade (Hint: that may not be long!)
Make sure you get all the peripherals you need: PSU (official one), HDMI and USB cables, keyboard, mouse and monitor if you don't already have them. And of course two or three SDCards (A1 16GB are best value at present, A2 doesn't help).
Hope you can get one soon, tell us about it here.