Newb question - keep it stock or mod?
 

I am purist at heart, but if you were starting from scratch with a nice FD with about 50K miles, would you leave it stock or are there some "must do" upgrades that would be smart for keeping the car in good condition?

There is no ‘right’ answer to that. And if you have emissions testing where you live that will dictate a lot of what you can do.
The farther you stray away from stock the less helpful the Factory Service Manual tends to be. And heavily modified cars tend to limit broad appeal at resale.
There are some basic reliability mods that most of us suggest...
*It’s rare these days but if your car still has the stock pre-cat, remove it and replace with a good quality down pipe. They can collapse and plug the main cat, which can damage the engine. Only good for cold start emissions anyway. Use header wrap or a heat coating on the downpipe to reduce under hood temps. BTW, this is a 2 for 1 because it’s also a performance mod.
*All-metal AST or delete properly.
*All-Metal radiator custom for the FD. Fluidyne and Koyo are two popular brands. You do NOT need aftermarket fans.
*At 50k you’re about due for a new MAZDA stock thermostat too. Good time to do it when coolant is drained.
*Install a boost gauge
*Install an aftermarket temp gauge. Do NOT rely on the stock gauge. It’s not linear and pretty worthless.
*Auxillary grounds. Think of a circle: Battery to engine, engine to chassis, chassis to battery. Be sure to check the stock exhaust system ground just aft of the main cat.
*If the car is new to you, change ALL fluids, brake, transmission and differential gear oil. Coolant can be done with the radiator. Refer to the Factory Service Manual for directions and specs...and search the 3rd Gen tech section for additional tips.
*New plugs and wires might also be good. NGK Factory heat ranges since you’re stock. NGK wires too.
*The FSM can be downloaded for free from the FAQ sticky if you don’t already have it.
*When you’re familiar with the car and depending on DIY skill level I recommend replacing the fan thermoswitch with the S5 FC version which lowers threshold temp for fan activation from 105 C. to 95 C.

Thanks @Sgtblue , great suggestions and quick overview! Now, to find the right car...