No modifications required to fit the T9 5 speed, as the kit comes with the bellhousing. BGH also has a "long 1st" closer ratio 'box that is far superior to the standard T9, with a useable 1st gear (longer, so you don't have to shift at 20 kph) and the gap closed between 2nd and 3rd (which is like having 20 extra HP). The car is very light, so you don't need the very short 1st gears of the sedan gearboxes.
What you find in these builds is that there are a lot of parts, even with a simple car as the W11. It is easy to get the car to the point where you can sit in it, and make engine noises... But there will still be a LOT of parts to attach. Working full time on the car (say, 6 hours a day), you can have the rolling chassis assembled in less than a week, including the brake system plumbing, provided you have all the bits (brakes, axle housing, spindles, king pins, finished and all the fasteners in hand).
But that's not the hard part. Once built, it is not really a car until it is sorted, and you have to drive it to get that part right. And then, you have to know what is wrong, and how to fix it to get your desired result. Fortunatly, we have an excellent community to work with.
I have built a few of these, so I have some good starting points as far as suspension settings (including corner weights), which are the hardest to sort out. A lot of W11s are not finished (but running) and are sold along, as it is often beyond most 1st time builders to make the car useful or reliable. No trick to this. I have over 50K on mine... Not enough parts to be unreliable, but it did take some sorting, even on my first; a fully built and driven W11 that was completely dreadful on delivery. Now, a paragon of virtue and reliability.
Sometimes, it is easier to wire the car from scratch than to use the wire harness that is available with the kit. Use a wire scheme from an old Lotus 7 S2 and the standard Lucas wiring color code and wire. After about 4 hours of wiring, and wondring what if you have the right idea, you will have an epiphany, and be able to wire the car using only the color code, and no schematic... What was just encrypted code will be crystal clear and second nature. Use a Painless Wiring fuse box, and you wont even crack look at the scheme. All you need is the color codes.
Then, there will be the mods. Lightweight, double pass radiators, the T9 trans mount, which gear stick (and short throw shifter) to use, sorting the brakes (using taper wheel bearings is the key to a solid pedal) Fabrication will slow you down, but can be a lot of fun.
And then, there is the reluctance to do the first start on that engine you rebuilt yourself. Just make sure that you have oil pressure prior to start, and that you have properly filled and leak tested the cooling system. I know what I am doing, and still, I might wait a week or so... Maybe it is burn out, or just fear. I double check a lot of stuff... Once found that I left the nut off of the clutch fork pivot but was able to install it without having to remove the engine... Lots of bolt tightening. Lots of questions..."Did I put the thrust bearings in correctly? Is the clutch disc 'wrong way ' round? Will the smoke escape from the wiring when I connect the battery?" That happened once. Wire was shorted on the panel of my Lotus 7 S1 restoration... 'Long time ago, that...
Think about what you are building, and think about what you have to do if you have to take it apart again. Thinking ahead will ensure that the car will be easier to service, especially if you are serious about sorting the car out. Make it easier to work on. Fewer fasteners. Make sure that the fasteners are turned the right way for access and torque.
It is incredibly rewarding once you have it on the road, and get the steering sorted, the dampers set, and the carburetor sorted. I have had mine over a decade, and still love to drive it almost daily.
Enjoy the build.