New DB01 Durga/Baldre coming, a few beginners questions though

[built2scale]

First, I am new to the hobby, well kind of, I have the experience of building a Tamiya 2wd many years ago. Always remembered it as a fun challenge. Picked up a Losi Mini-T a year ago but sold it because I felt it was too small for backyard bashing for my son who's 7.

I'm currently reading the main thread and I'm gathering some info for when my Baldre arrives. I want to keep it as stock as possible but I've got the following in mind as hop ups.

Tony Screws Set
54017 Front One Way
54028 Damper Set
54018 or 54061 Slipper Clutch

One question I have is whether to go with the single slipper clutch or the double. I'm leaning towards the single because it costs less but it's not like the double is going to break the bank. I just would like to know if the double is worth it's cost.

For the hex screws, what is the driver size I would need, is 2mm correct?

Planning to go with the Novak Havoc brushless esc and motor. It's the least expensive one I can grab, hoping that would fit the DB01 ok.

Any suggestion for a inexpensive servo brand/model? FWIW, I'm in the USA.