£10,000-£12,000 to spend, what do I get?

I'm currently looking to expand my setup into a studio to rent out and charge for mixes. I have a PRS Archon 100W Head, a Mesa Rectifier 2x12 cab, a Mac Pro, a Focusrite 2i2 (need to upgrade this, was thinking maybe Apollo 8P or something?) and I'm looking for acoustic treatment here in the UK.

Thanks!

Edit: I have a pair of KRK RP8 G3's

Hey MattWard,
I'd buy some good monitors, room treatment and an interface that suits your needs. If you have some money left, it's always nice to have a good mic and preamp. But that's me.

I think that this money will be in good hands. My money is about the same. I still use it.