h/c/i = heads, cam, intake
suspension- first sub frame connectors. replace worn sway bar links with poly. If your stock rear lower or
upper control arms or bushings are gone, then replace. I prefer Maximum Motorsports pieces. Shocks- best setup for a DD is front coilovers and a good set of rear shock with springs. Most guys run a good set of aftermarket springs and shocks. The brands recommended are going to be based on personal opinion, price, and experience.
I've run bilstein shocks, Tokico 5 way ilumina, and currently Tokico HP (blue shocks). For springs, I have run
eibach pro kit, drag kit, H&R, Steeda.. I like for a DD the H&R or STeeda springs. If you don't want to much lower than stock, you can run late model stock Bullit or Mach 1 springs. General rule is if you lower the car more than 1", either a bumpsteer kit or CC plates are needed- MM makes the best IMO but guys do like
J&M, Team Z.
Brakes- that discussion can take up an entire book. Depends on what you want and how much you want to spend, ranging from upgrading your 4 lug to a full blown 5 lug Bear or Wilwood setup. For a good cheap upgrade, again MM sells a nice kit for about $250 that included SS lines, SS caliper pins, and upgraded pads and shoes. I've put that kit on two cars and it makes a noticeable difference.
GT40 heads- iron- have been a tried and true budget upgrade for years. Most guys get the entire top end- intake, heads, TB from a mid 90's explorer motor, as that is essentially the GT40 top end. That's what I have. Cheap, for under a grand you can get around 280 hp with a good cam and that setup. If however you are not on a budget I highly recommend consulting someone like Ed Curtis or Buddy Rawls and they work with you on a custom cam, the proper alum heads- springs etc.