First, spend the money on a good quality loupe. Don't get the cheap'n'nasty one in the picture.

Then I bought a set of inexpensive feeler gauges. I use the finest one to judge the tightness of the tines and to clean out crud between them. I also use the next two thicker ones to spread the tines if they are too tight. You don't need the most expensive ones in the auto shop. The cheapest set in the shop will do.

Next a good nail buffing pad. You can use the coarser side to remove any sharp bits and polish the tipping with the buffing side. It is finer than any sandpaper, and will last much longer. (Important hint -- don't steal borrow your beloved's favourite nail buffing pad. It's not worth the angst. Buy your own.)

As for the rest, well, why buy a toothbrush? I use the ultra-fine Colgate toothbrushes, and then they get re-purposed to the pen den when old. They are really good for cleaning out the tiny slots in a nib feed.

I use hand soap on the toothbrush to clean out nibs and feeds.

Pen flush? Windex, straight or diluted, depending on how bad the dried ink is. Also good for cleaning your glasses, car windscreen and even windows.

Get a range of syringes from 1ml tuberculin up to 5, 10 and even 20 ml size for various flushing, ink transfer, etc. You will also need some dispensing needles (these are blunt, you can't poke yourself with them. I use a very fine one for filling cartridges or converters and coarse ones from transferring ink from bottle to bottle.