Hello, iam using midi to guitar on my standalone mac and i am very satified about it.
I also like to use it in the nearby future on an iPad. I dont want to spend al lot off money to buy a new IPad. So my question is the ios app version is working good on a iPad 2018 or can somebody tell me which iPad versions it is working well, Thanks in advantage 
the current version runs on iOS 12 and up, and will work on any ipad or iphone that supports iOS12. This automatically prevents use on too old ipads/iphones with too little CPU power.
An 2018 ipad will work fine.
The only differnece between older and newer ipads is, that you might run fatter synths on the newer ones before reaching CPU limit. The builtin little synths in the app will never reach the cpu limit, even on the oldest machines.
Note: this was all talking about using iOS MG standalone: when you use elaborate setups with AUM, Audiobus, mulitple synths etc, users often feel that they need the fastest device possible.
Thanx 4 your quick reply, greetings :-)