Mine's branded "Scorpion" and I just caught where yours is the 25g versus mine which is 15g. It looks like there are actually very few truly different 15 and 25 gallon tanks (probably all made by one or two companies). Sounds like you got the right plan thought out and given you will be lugging ~200lb loaded, spending <$150 for dependable, ready made solution for the carrier is the good choice.
BTW - I vaguely remember seeing a catalog where you could buy all the various individual sprayer accessories, including a 5' offset boom for not much money. Might have been New Harbor Feight. If cheap enough a larger folding boom might also work whereby you keep one side capped off and/or cut off/folded in. The side benefit is you can then fold in your offset boom side for easy storage. Incremental cost wasn't that much more.
Finally, being able to use the 3pt to raise/lower a bit will allow you to adjust spray pattern to get more width out of a 5' spray boom.