I Got Funding Approved for a ISX Powered Tractor

[JMBT]

Well first of all congratulations!!!, and welcome to the O/O world! By taking the step to add a bit more responsibility to your job, you have opened the door to much, much more potential revenue!! The game now is not how many miles you can drive, but how profitable you can make the miles that you do drive!
I can't give you any opinion on the 587 Pete, I have no experience there, but with 3:42 rears you may be geared a bit high depending on your targeted cruising speed. There is a ton of info and many opinions on this forum about how to drive so you will have to read some and decide that for yourself, but keep in mind, slower speeds mean more $$$ in your pocket, and therefor less miles that you have to drive to keep the same amount of money.
Since you have an ex-company truck, you have to assume maintenance has been neglected, so I would start by first doing Rawze's full EGR tune up, as soon as possible. Watch his video series and do as he instructs. Doing this may also let you know if there are other emissions problems in your near future. Second, do an overhead and look closely for cam wear and other signs of premature wear from extended oil change intervals. Third, buy a Chinese insite knockoff from eBay, and become very familiar with the software. You will live and die by it unless you want to give all your hard earned $$$ to the Stealerships.
As far as the ECM, look at the current calibration number and make sure it matches the one on the data plate on your motor. If it doesn't you can post the two and Rawze or some of the other guys can tell if it is compatible. Look at all the engine abuse history information. It will show you time with coolant over temp, oil over temp, engine over speed, coolant level low,...and so on. That will let you know how rough it has been driven. You can also go into the trip info tab and see how much idle time, etc the truck has. Another thing to look at is the regen history, it will show you the last 10 regens the truck has done. Note the engine hours between each one to see how frequently it is regening, and the max inlet and outlet temp, a large temp difference can mean a restricted DOC or DPF that needs attention.
I know it may all seem a bit overwhelming, but it is definitely worth it in the long run! Congrats. again and welcome to O/O trucking!!