GT:
If you are interested in TBones, there is a book you should get. Can't recall the name of the book, but it is written by Dick Ward and will answer all your questions and more. Dick was a long time TBone pilot that died last year. As pointed out earlier in the post, you can get one with the 400hp Lyc. and the geared engines (some supercharged). Dick does a good job of going through the various models and options that were available. I think his book is advertised on Barnstormers (just go through the TBone section). If you can't find it there, the Beach society would probably have a copy.
I would highly recommend getting in touch with someone who knows what they are doing around TBones, if you are intent on buying one. There are some really good deals and there are some money pits.
I have not owned a TBone, but have been around them some and they are awesome airplanes. They are tough and I believe that there has been almost no structural failures in them (the book details that). I know of one plane that was based out of a grass field (read pasture conditions), and it did fine. If you get one with the airstair door configuration, you can actually get up and move around the cabin during flight. They also have a great useful load capacity. If you get the supercharged engines or the 400hp Lycomings, they have pretty good single engine altitude capabilities. They are very straight forward and a very good platform for single engine IFR.
That said, your fuel burn will not be a whole lot less than a D18 (some but not a lot). The T Bone and the D18 will cruise about the same. Operational costs will also probably be slightly less, but I wouldn't count on them being a lot less. Parts are actually more available for the geared engines than you might think, but they aren't cheap.
Final comment, if you do go with the geared engines, you have to treat them right. They don't take as much abuse as a nongeared engine. The book goes through this in pretty good detail.
In the final analysis, my suggestion would be to fly less and do it behind the round engines!! You just can't beat the roar of those radials and I have seen some pretty good deals on D18s lately.
Happy hunting!!