I use an IK multimedia iRig Keys with an iPad. I have various apps including Model D (MiniMoog), iMS20 (Korg MS20), ODYSSEi (ARP Odessey) & Yonnac Kauldron which is great for funny noises. For polyphonic sounds I use iM1 (Korg M1) & Galileo (Hammond B3 Organ). If you already have a decent iPad (or iPhone) this might be the most cost effective approach.