I have found that architects and/or draftsmen seem to pick up CAD quicker and more efficiently if they draw ordinary lines and circles when beginning CAD as opposed to using a BIM or modeling technique you see in products like Revit or even Softplan. You have been drawing primitive lines for all these years, so doing so electronically will ease you into the digital era. Once you have learned the ins and outs of the digital workflow, you could then advance further into BIM or modeling programs that take advantage of your newly acquired knowledge.
I have witnessed several small architectural firms waste thousands of dollars on software licenses that go unused after trying to leave the old fashioned drawing boards. I think that in many cases, they were trying to learn too many things at once, and that is why they ultimately failed. The biggest hurdle, in my opinion, is becoming used to producing a full set of plans electronically - it's kind of a mindset you need to acquire. Once you have that down, learning the specific syntax of one piece of software or another is easy.
I would recommend Autocad LT for you, as it could do everything you need. But if you have the funds available, a product like Autocad Architecture would allow future expansion for modeling and customization once you become proficient. Intellicad wouldn't be a bad choice either if you were trying to save some money. Of course this is just my opinion - you may find otherwise. We have had licenses of Softplan, Chief Architect, Datacad, and Archicad throughout the years, but in our case they are all sitting in a cabinet somewhere not being used now. For the most part, they are all fully capable but we just can't depart from Autocad.
Also I would steer way clear of Rhino in your case....it's not a bad program but in all honesty it probably shouldn't have even been mentioned in this realm