Howsit, I have the same exact spec however in Steptronic. Pricing on m135is and M140is have definitely turned after the new model released with a 4 cylinder engine. 
When I was looking I was also very however there is a ton more to consider...I overlooked the ones which already had preventative maintenance on it and went for a dealership buy...they did bugger all maintenance on the car and I had to do everything excluding mods. It costs a pretty penny to do that especially with the right people and price as well.
Also, many 2013 models are very well specced...they come with xenons, suroof, ProNav, HK sound and some with heated seats or rear view camera too. PWG is seen to be a 'slower' car and bla bla bla but its more fun with the lower torque if you playing around in-town mostly. 
Either way, you will have to consider the cars are in demand more so than the highly priced 4 cylinder variant, if the car has had all preventative maintenance done then the owner wont throw his cash away either and is in good nick with low mileage, be prepared to pay a good price for it cause then it becomes a get in and drive situation with a fun, reliable car. OR buy one from a dealer, and drive that journey of maintenance and mods...maintenance which will cost more of course and learning of how and what to do.
Good luck with the search bud, a manual M135i would definitely be a ton of fun!!! but that 8 speed steptronic box is such a gem!