Hi Tony

For £1500 you can have nice PC which will perform nicely in any program,I personally would recommend build yours own PC as above you will save lots of money and yours PC will tailored to yours needs

I would go with something like this:

http://uk.pcpartpicker.com/p/wxG33C

I've build several PC and always I would go with custom PC,there is lots of room for customization and mainly they're not build with cheap parts as those from DELL or many other companies

Yes and deffo I can recommend multiple display setup,I currently running two monitor setup which is awesome,although still thinking go with 3 monitor setup

Assume  you are in UK?

Hope this help

Thanks,Jura