If you go the direct fire route, it really needs to be automated/semi automated. Take a looks at the original Brutis 10 Build , or the build I primarily copied by Jlandin and here is my build. Like you said you already have the burner and a single tier ready to go. Just get a quality false bottom from someone like Jay at Nor-Cal Brewing Soultions, re-circulate while you mash and you will be good to go. With my system, like Jlandins I can keep my mash temp w/in .8 deg F with no issues, and never have a scorching issue.
Can't really comment on the rims tube, however many many people use it on HBT and love how it works. Like you I just had the gear ready to go for a direct fired system so I went with that:-)
As I am sure you will find on here there is about a million ways to "skin a cat" and the goal is to find what works for you and stick with it:-)
BTW took a look at your build here Looks really great man, real solid and clean;-) If I may suggest something, after you fire it up and if you have issues with the burner flames being too sooty/poor flame and or/really heating the S&*t out of the frame, you might find that even with your spacers you might need to cut vents in the back side of the bottom skirts of the keggles. I have plenty of space on mine and had issues, after cutting vents, my flame, stand temp, and over all heating efficiency were good to go.
Overall great job and let us know what you decide to do. I am sure there are som RIMS/HERMS guys that will chime in:-)