mmmm... I have to agree for the most part. Mainly in the areas of getting things squared away before you turn to modifications.
As for the OD or 5-Speed conversion question though, I'd have to say go with what ever is easier for you to obtain. An OD from a MGB parts car may keep the car closer to stock or at least in OEM trim, but it may be a bit more difficult to find. On the other hand the Moss Motors 5-speed conversion is a bit pricey, and you might be able to get 2 MGB OD transmissions for the price of Moss's 5-Speed kit. Between the two routes, I have to say go with what your wallet will support.
As an aside, Moss Motors isn't the only company that sells 5-speed conversion kits for MGBs. I know of at least two other kits that can be found on the net. One of them uses a 5-speed from a Nissan 280z (I think), the other uses a 5-speed from a Ford Sierra(?).