You need FL22 coolant, 2 x 5 litre containers, capacities in the link below.
Thermostat wise there are a few on offer, last time I looked from around £20 upwards, the genuine Mazda thermostats are quite pricey from Mx5parts.
If you search the forum using the search function using something like NC thermostat change it’ll give you an idea what’s involved.
The thermostat is housed under the intake side of the engine, associated hoses and throttle body needs removing to gain access. Flush the system out with deionised water before filling with fresh coolant👍
Probably more than 6 litres, it depends how much you extract from the system when flushed.
Link below, I’d probably buy this one for the stat. As above advice, if the neck is long on the stat, trim the hose about 30mm (large one) on the car for a snug fit. That’s how I fitted mine.
You may want to have a look at Ravenol which is a concentrate and is fully compatible. With this you can add sufficent to maintain the coolant strength without worrying how much of the flushing water or indeed the original coolant remains in the system. Dont forget that there is a bleeding procedure once the system is filled. A quick search on this site should pull up an excellent post of what you need to do.