I just started working remotely and am suffering from horrible WiFi on top of slow speeds at the gateway. I know a Deco mesh system can help the first problem; only changing ISP can help the second. 

I am in a 2-story townhouse, about 1400 sq ft total, so it's a small footprint, and my office is as far as possible from my gateway, probably 35-ish feet including going through the floor/ceiling. 
I need the best bang for the buck, as my funds are limited, but given that my neighborhood only currently gets speeds of 50-60, I don't need gig speed compatibility. 

I've seen several Deco systems on sale at WM and AZ (not sure if I'm allowed to spell out store names). I've also read that some systems can cover my square footage with just 1 device, but I've no clue if a 1-device set up would help at all. 

Some possible setups I've seen and can afford are  2-device M5, single device X20 or X55, or 2- or 3-device S4. 

Given a budget of around $100, the limitations of my current incoming speed (ISP competitors might only get me to 100 speed for more $$), and a 2-story townhouse small footprint ~1400 sq ft, which Deco would be best?

thanks in advance!

EDIT: I would not be using Ethernet to connect floor 1 to floor 2, so I need wireless connectivity.