RR & AR, you might consider bumping up your base repair
tools to bring them in par with your other tools. (Welding vs Ptex candles.)
As Nefwydog says, your time is a real consideration when considering (or rationalizing) the purchasing of tools as is quality, life span, performance level and 'capability empowerment'.....and economics. If you can eke how better performance and longer life spans of your gear, they easily pay for themselves.
(AR, I hope you will be receiving some form of compensation if you take care of your friend's skis with your tools and time.
Regarding brushes, I have a full set of manual brushes (because I can) and do find them each nice to have, but I could get by fine with 3 or 4. Stiff metal for aggressive cleaning, structure refreshing and additions (post bass repairs), hard & very hard wax like the Maplus RB hard. Soft nylon for general purposes and polishing. Hard horsehair for general structure freeing. Stiff brass for warm and hard waxes and wax removal. Soft brass as a frequent use for structure freeing brush. Hard nylon as another universal brush option for polishing and brushing down bases. I also have a dedicated 4" x 1 1/2" stiff nylon brush that sits in my stand, bristles up, for quick, easy & frequent stone and file cleaning and other cleaning tasks.
Thought the ovals are definitely nicer and faster than the rectangular brushes, I clearly prefer my roto brushes as they save enormous amounts of time and effort, provide great results, and offer a broad range of capability with a few brushes and variable pressure and drill speeds: nylon, stiff HH & brass, along with a cork.
For many though, a low cost approach to purchasing tools and gradually adding to your kit as you gain experience and mojo, is very practical. Starting out with tools that provide multiple capabilities and options (ie multi-bevel guides), until you get a clearer understanding, needs and goals, without busting the budget is part of the learning curve and process.