if you had to pick one, which would you get? buying advice

[Perfectlap]

yes you can do just springs, although I don't think its a good move on a car with high mileage shocks.
My old mechanic refused to do it on a previous car. He knew I would not be happy with even less travel on an already low sitting car and he dreaded the idea of having to charge me to undo it.

As for the choices.

I think side skirts is by the far the best bang for the buck mod I have splurged on.
With 18" or larger wheels, side skirting really balances out the lower aspect of the car. Makes the car look bigger and wider, less roadstery if you will.
LItronics being the worst bang for the buck.
Bumperettes are nice if you can get them cheap, and if you never park your car publicly. Those rubber pegs have saved me many times.
I don't care for the difusers, makes it too busy for my taste. adding a Euro spec rear bumper seems like a better mod for $1200 or so.