Yes, adding a barlow lens would increase your focal length. I haven't done any long exposure imaging with a barlow yet, so I can't really comment on that.
The mounts you are looking at are definitely capable of giving you decent guiding at 1200mm, though. They also have all of the necessary functions needed to do a star alignment and polar alignment.
If you want to do long exposure imaging without a laptop etc., there are some standalone auto-guiding solutions out there. The only one I am familiar with is the Celestron offering. I know QHY-CCD had one in the works but it appears that it hasn't left the stable yet.
Information on the Celestron NexGuide unit can be found here:
http://www.celestron.com/browse-shop...ide-autoguider
I've never used one personally but perhaps some other members can offer their testimonials or suggest alternatives.