Do not get an online MTM sport coat. Shirts I understand since trial and error is pretty cheap if you mess up the measurements or something gets lost in translation between you and the company. For any jacket, however, your trial and error process will be more expensive and there are more things that could go wrong. It's also easier to fix a badly fitting shirt than a badly fitting jacket in general. I agree with the suggestion of getting RTW tailored vs. online MTM.
If you are going to do MTM, you really should have someone measure you. Travelling Hong Kong tailors will do this as will brick and mortar MTM places such as Brooks Brothers. It sounds like you might end up spending more than you'd like going this route.
For me, the value in MTM is greater fabric selection vs. RTW, ability to make stylistic choices such as number of buttons, lapel type, etc. and a better fit than RTW when it's done well. It can also be way cheaper than bespoke. I'm not in a position to be spending $4,000 or so on a suit but if I can get a MTM garment for $1,000 or under, that makes a lot more sense.