Katie, there's other AT&T MVNOs that do PAYGO without the USSD balance notifications (but it is very common with AT&T MVNOs). Although Airvoice and H2O Wireless do this with their PAYGO accounts, Puretalk USA does not. At least under Android, there's also apps that can suppress USSD notices and/or convert them into notifications, but some of them work better on older Android builds than newer and others require Root. Right now,
this one seems the most reliable over on the Play Store, but it costs one buck, I've never personally used it, and as such cannot vouch for it. It's also worth noting that most T-Mobile MVNOs don't bother with USSD notifications, so keep that in mind as well if T-Mo coverage is good in your area.
mVoIP providers are pretty terrible service wise, and any provider that forces hardware lock-in, terrible contracts, and doesn't provide any real customer support is a raw deal no matter how cheap it appears. There are better out there for less if you just exercise moderation, and if you were able to get away with the Airvoice $10 plan in the past and only switched away from it to the unlimited plan to only avoid USSD messages, that seals it as a slam dunk for getting better for the same amount or less.
Regarding phones, if you're wanting to go smartphone, there's plenty of cheap and used on the market well under $100
if you know how to shop for 'em. It's also more environmentally friendly and considerate to the state of rare earth metal mining. However, if you feel compelled to buy new, Look into Blu handsets. They have a lot of really cheap, nice, carrier unlocked stock Android handsets for under $100 that also have user replaceable batteries (which is increasingly rare, especially at these price points) - camera quality is the only thing that seems to suffer. The Blu Life Play Mini seems to be a pretty sweet deal for $70 off of Amazon.
I know you're wanting someone to just tell you what to buy, but you should put in the effort and make sure to buy what you need instead of letting other people spend your money for you.