I agree with Diki.., the PA-588 route will be expensive as hell.., AND the 588 won't be something you'll wanna haul around from gig to gig either. If there was an 88 key speakerless version it would make more sense, but the addition of the speakers with amps ect have added to the weight.

Yamaha actually does make an 88 key arranger keyboard, but the problem is that it's not the traditional arranger nor does it fall under the arranager category that the typical arranger player would buy (as it's limited in the arranger section) but still at the core it's an arranger. That keyboard is the Yamaha MM8. It's an 88 key version of the MM6. Both boards are arrangers, but are not as as full featured as Yamaha's other more traditional arranger keyboards.

I would buy an inexepensive 88 key controller as suggested by Diki.