My Miata recap:
All the old 1.6 & 1.8's are pretty gutless cars. The lack of torque is such that when I've switched mine from the little stock wheels to the 15x7's with Azenis for auto-x events, I can noticeably feel less acceleration from the extra rotating mass of the heavier wheel/tire.
That being said, with decent rubber on you can throw it around almost like a go-kart, so it's still fun.
The engines are pretty reliable. Mine has ~165k miles and still runs strong (for a Miata) with good oil pressure. Parts are pretty cheap, since everything is pretty small and light.
The most expensive hard tops are the OE ones, since they are required for Spec Miata and therefore sought after by every SM racer building a car. Aftermarket might be cheaper since they are disallowed by the SM rules.
The early cars are also definitely flexi-fliers. When you put on a hard top, just that plastic/composite/whateverthehellitis top with its 4 clamped (not even bolted) connections very noticeably stiffens the chassis.
The first couple years of the 1.6 had a much-talked-about-on-the-internet crankshaft keyway issue. Personally I think the level of angst over it is overblown. I have a 1990 1.6 and disassembled everything to check out the crank nose; it was flawless. Take that for what it's worth.
As far as adding power, forget it unless you force some extra air in there with a hamster wheel or drop in a real engine.
As far as braking, they have good OE brakes. Use more aggressive pads and it'll stop *hard* on good rubber. Maybe add braided lines for a little extra pedal stiffness if you want, but it isn't a huge change.
As far as handling, they're very well balanced right out of the box. I went from owning a 5.0 Mustang to the Miata, so they were polar opposites. The Mustang was a bear; heavy, slow to turn in, understeered excessively with the occasional snap oversteer to liven things up - not much fun to drive through corners. The Miata will turn right in and go wherever you point it. When you take it through a turn it just responds like a car should when cornering, with little effort on your part. Only drawback is body roll is excessive with the OE suspension when you push the car. Add some stiffer swaybars and it will feel great.
I'm gonna get rid of mine soon so I can do some stuff to my Jeep, but it was a fun car for a few years.