I've been doing iPad mixing for a few years and wouldn't go back; the joy of fiddling with levels while buying a pint at the bar, or having a pee is entirely liberating
I started using iPad with the Soundcraft Expression 1; good option if you want a traditional desk format with added iPad. Then moved onto Soundcraft Ui16, Behringer XR18 and now X32 core. I reckon best bang for your buck is the XR18 (I wouldn't go for the small XRs as they don't do multichannel recording). But would probably suggest external router for stable connection. As well as freedom for the engineer, they allow musicians to set their own monitor levels. Much easier to do in ear mixes. Loads of effect processing options. The learning curve can be steep, but not too bad if you're used to using DAWs to record music. It is an expensive endeavour, but pros hugely outweigh cons in my mind.