Regardless the pressure inside your PC (positive, negative or neutral), you still need fan filters to avoid dust buildup inside the PC, since air is going into the PC case and also out of the PC case.
As far as fan amount goes, that depends on the GPU you go for. For low heat output GPU, e.g GTX 1050 Ti, the 2x stock fans case comes with would be enough cooling wise. But if you're putting e.g GTX 1080 Ti into there which has aftermarket cooler as well (not blower-type), then it's high heat production could rise the PC case internal temps quite high and 2x fans may not be enough.
Here, try with 2x stock fans first. If temps get too high, buy additional fans.
Demciflex filters that i bought are specifically made for my Corsair cases and i bought them as a set. My Corsair 760T filter set costed €61.10 and my Corsair 750D AF Edition filter set was €56.00. Since there is no set for Meshify C Mini, you need to buy regular Demciflex filter. E.g standard 120mm filter is about $5,
link:
https://www.demcifilter.com/demciflex-magnetic-dust-filter-for-120mm-square-computer-dust-filter
You only need to buy bottom filter since there's no point to filter exhaust air at the top and rear of the case. For front filter, there is a topic in the forums where there was discussed about replacing the stock foam filter. It's a good read if you're also planning to modify the front filter and make it better,
link:
http://www.tomshardware.co.uk/answers/id-3785938/aftermarket-front-dust-filter-meshify.html
Cleaning the Demciflex filter is easy. I rinse it in slow flowing cold water and afterwards i gently tap the filter dry with towel. There's also a guide on how to clean Demciflex filters,
link:
https://www.demcifilter.com/cleaning-2
As far as cleaning the stock front foam filter goes, things get difficult. Since foam filter catches dust inside the foam, cleaning it is very hard. You can try using vacuum cleaner on it but i, personally, would avoid foam filter all together and replace it with something better. The topic link i gave above discusses about replacing the foam filter.