Getting started with Mindstorms and/or Technic
In my opinion, Mindstorms is a subset of the Technic line up (Like how Police is a subset of Lego City). So most of the parts are interchangeable. The only part incompatibility issue you will come across is the electronics.

I would say go with Technic first. Learning how to build studless and basic Lego mechanisms will be more useful than robotics (initially). Buy the bigger sets as they have more parts and lower price per part (typically). 

50-100 quid a month is quite healthy. Once established, Lego has a pretty low maintenance cost (If you don't want every set). You will find Brick Owl and Brick Link a good source of specific parts.

I suggest a browse of http://sariel.pl/ and http://www.technicbricks.com/ to get some ideas on what is possible.