Surely you've made your decision by now and have already started down your path, but I'll add my experience as one additional data point, and because I'm bored at work.
I bought my '08 135 this summer, July 2018. 103K miles, bone-stock, original owner, an older gentleman in SoCal. The car drove perfectly fine the first month, and I went FBO by the second month. It was pretty fun! Until my turbos started smoking.... At this point in time, I'd guess that a lot of these cars are around the 100K mile mark, and probably not ready to handle 50% more power output, especially the turbos. So i wasn't too surprised about them smoking, but didn't expect it THAT soon. At least it was early enough that, if i did upgrade them, I'd get to enjoy them for as long as I own the car. (as opposed to having to replace them right before selling the car). My indy shop said they might last a while if I just baby the car, but they're definitely on their way out. I wanted to enjoy the car, and not worry about failure every single mile, so....
I opted for Pure S2. (they went on sale, 15% off, a month later, around Black Friday.
) $1,200 for labor, $500 tune, and a few hundred for the LPFP and misc stuff.
I took it to its first dyno event a couple weeks later, and only made 357hp.
The dyno graph was strong and smooth until 80mph, and then it fell flat, and became very jagged between 80-150ish. Back to the shop it went. Apparently it just needed a new fuel pressure sensor, as suggested during the turbo install. $250 and Done. (Again, expected, but not sooo soon!)
Took it to another dyno event the following weekend. 350hp....
. Back to the shop.
Everything was logged and 'test-driven' and no issues were found.
At this point, I'd given up and just accepted my fate. I rarely go over 80 anyway. Big city livin' for ya...
Less than a month later, my car starts stuttering during very gentle weekend driving with the misses. After a couple of red-light restarts, it just dies completely. It's towed to the shop. Turns out the HPFP decided to retire. (Side bar. It was replaced at 17K miles, and again at 29K miles, last in June 2008. I called the dealer to see if they'd replace it since its a known-faulty part and the warranty was extended to 120K miles, but apparently its also a 10 year warranty, so I missed it, almost barely. womp womp.
)
Moral of the story is this. Yes, you can surely do everything on a budget, and piece it all together slowly, as needed. And Yes, $5K is enough to buy the turbos and install them, plus tune and fix your gasket/seals. But it's also definitely inconvenient. You already saved up $5k. Just save up another $500-$1K and do it right the first time. All the issues I had were expected. They're known issues, even with just FBO, let alone S2 turbos. I didn't save any money by waiting an extra month to replace the fuel sensor or upgrade the fuel pump. If anything, it's an extra few days of uber/rental cars, and tow trucks (insurance FTW, even though they wanted me to wait 3 hours for a tow. I called my own random company, and am being reimbursed).
Just save up and do it right the first time. You're already so close! The peace of mind that comes with it is priceless. Not to mention the convenience. If nothing else, you might save a couple hundred doing everything at once.
Would you look at the time!!??? Time to go home!