Do not buy that phone, get yourself a Motorola G, amazon had the latest version up for sale for $150.
If ynab isn't on the amazon appstore, you'll have to side load it. I think it would be better for you to get another phone...
Edit: it looks like it's back to the original price ($179), Motorola Moto G (2nd generation) - Global GSM - Unlocked - 8GB Black by Motorola http://www.amazon.com/dp/B00MWI4HW0/ref=cm_sw_r_udp_awd_q6eIub1JBM0ZT
I think either the global or US version should work here but double check that!
anyway i was searching for some phones (took me some time)and i think these might be of interest to you
i went for max battery life, double sim for adaptability, 4 cores or two and 4-2 gb of ram for speed obviously 300$ tops for price
Moto G 2nd Generation-Global GSM-172$!
Those are the ones that i liked. dont know if they look good to you but they are really functional!
BTW do you have a wireless router? its a must with this things