MacBook Pro + 8GB RAM (don't buy from Apple) + SSD + large external monitor is a good setup. Another option I would suggest is looking into a Mac Mini, which you can just plug into your existing stuff.
Do not get an iMac whatever you do. They have a design/manufacturing flaw in the screen that lets dust/dirt/fumes between the panel and the backlight causing the screen to appear dirty or burned. This is a very common issue and there is no fix outside of tearing the whole screen apart and cleaning it (which is very hard to do and very likely to break it).