The current release of SongBook+ runs on all iPads since iPad 2 and newer, also on iPad mini and iPad pro (both sizes) models.
The size of built-in flash memory (16 GB, 32 GB, 64 GB, 128GB, 256 GB,...) is not that important if you don't want to store very many photos, music or videos on iPad or install very many apps. Also games may use much memory. The PDF files used for SongBook+ don't need that much memory.
On the elderly iPad 2 (from 2011) SongBook+ may be a bit slow, but it still works without problems. The iPad 2 doesn't have a "Retina" display yet and therefore a smaller screen resolution and a less sharp picture.
Newer models of iPad have faster processors, more working memory (RAM) and thence are faster.
The current version of SongBook+ requires a minimal operating system version iOS 9.3.5 (the last 9er release of iOS).
Unfortunately it is not possible to install the latest iPadOS 13 release on some older iPad versions. If you want to use the latest iPadOS release 13 then don't buy an iPad 2, iPad 3, iPad 4 or iPad mini 1, mini 2, mini 3 or iPad Air 1.
When you plan to use external Bluetooth equipment (e.g. foot pedals) be aware that older iPad versions (the ones with old 32 pin connector) don't support "Bluetooth LE" (=Bluetooth Low Energy) and therefore don't support some newer Bluetooth devices.
If you still own an old iPad 1 (from 2010) and want to use SongBook+ on it, that is still possible but due to the old operation version system you can only install an older Version of SongBook+ (V2.1).