Suspension lift kits are the most expensive due to the parts involved. I know there is a body lift kit available for the 00-01 Sport Trac for around $300. Body lifts are easier, and you can use the stock suspension components. The downside of course besides finding one compatible for your '05, is that the frame will be exposed more to side collisions (not a huge deal, but still), and you'll notice more body roll, unlike a suspension lift kit which adds stiffer shocks and other components.
I'm not sure if the '05 have torsion bar suspension or not up front, but if it does you can to what's called a T/S lift, or torsion bar crank and shackle lift. Turning the torsion bars (bolt on bottom of frame, one turn is about 1/4" lift) and adding 2" shackles to the rear can give you a relatively cheap way to add a couple inches without adversely affecting ride quality. I believe for '04 the ST went to IFS suspension, in this case you'll be outta luck.
Body lifts are the cheapest way to get the most height (3" in most cases): PerformanceLifts.com -- PA 01-05 Ford Explorer Sport Trac 3" Body Lift Kit
I wouldn't order it online, I'd order over the phone just for vehicle fitment verification purposes.
Install for a body lift is moderate to intermediate, in terms of knowledge and tools required. It's best to use a large hydraulic floor jack, and several axle stands. Remove the body mounts on ONE side only and replace with the 3" mounts from the lift kit, then do the same to the other side. This keeps the vehicle geometry straight and true. There are a lot more steps involved, but that's just something to think about if you plan to tackle it yourself. I'm sure a good shop would charge at least $500 for the install. At least it's not as much for a suspension lift kit. Skyjacker is one of the better ones, but as you mentioned, prices aren't cheap; not only to buy the kit, but to have it installed.
body lifts will leave things more exposed, so be sure to order larger moulded 'gap guards' to protect vital components from dirt/sand, etc.
Depending on your current tire size, you can run up to 33" tires with a 3" body lift. Stock tires are in the 30" range (overall diameter that is). There are two things you have to keep in mind, overall height, and overall width. I would assume that a 265 tire is your best bet for maximum width. You could run a 285 tire but would run the risk of rubbing at full lock.
245/70R16 = 29.5" OD
265/70R16 = 30.6" OD
I'd personally go for a more manageable size, that'll allow some articulation. That's also if you plan to use your stock rims, which I'm guessing are 16":
265/75R16 = 31.6" OD
It's not really a matter of size, it's overall performance that you want. A large tire is heavier, especially in LT trim, versus P rated, for passenger/highway use. A 32" LT tire can weigh upwards of 60lbs depending on tread and build design. A 31.6" A/T rated 'P' can be 20-25lbs less! If you plan using 4wd extensively, I wouldn't exceed a 32" size only because the front differential isn't designed to move that type of mass.
A 2" larger tire will give you about 1" added ground clearance from the axles, but neither a suspension or body lift will increase ground clearance. It'll make it look taller and allow for better wheel articulation, so again, if you want the lift, I'd go with the 3" body lift and 31.6" tires (265/75R16) on your stock rims. Also with stock rims I wouldn't use a 285 tire, as it wouldn't be too safe unless Ford OK's the use.
If your not sure about the look you want, don't be afraid to shop around for used tires! there are a LOT out there. eBay Classifieds (Kijiji) - Post & Search Free Local Classified Ads.
is a good place to check locally for things. keep us posted on your progess!