You don't need a beer engine to serve cask conditioned beers - beer festivals and even some pubs serve directly from spigots on casks under gravity.
I've got a bitter cask conditioned in a 20L Speidel Fermenter at the moment (which is pretty similar to the pressure barrels you will see on UK brewing forums and shop websites). I gravity serve from the spigot on the Speidel, but you could easily hook up a beer engine to the outlet.
I replaced the airlock with a second spigot, which seals the Speidel (reportedly it's good to about 5 psi), and primed the cask for 1.5 vols C02 (about 1.5 oz corn sugar for my fermentation schedule). Let it condition at 65F for a couple of days, before cooling to 55F and conditioning further for a week.
I hooked up a cask breather to the top spigot to prevent spoilage, although I did draw a couple of pints off before this, allowing air in through the top spigot. A little controlled oxygen exposure is helpful for English style bitters, as it softens the bitterness a little. The cask breather is a low-pressure propane regulator (as seen on the link above) - fixed at 0.4 psi, and about $15 online - fitted inline in some 3/8" ID tube between the cask and one of my regulators for the kegerator (set at about 10 psi as it also feeds a keg of APA). Just in case of leaks from the Speidel I shut the CO2 off when I'm not serving.
This gives you a gravity cask setup for about $65-70. A cheaper option are the 5 gallon cubitainers that Northern Brewer sold for this purpose (out of stock last time I checked), which you don't need to worry about venting air or CO2 into to carry on serving, as their collapse as beer is removed, but these are going to look a little ugly for gravity serving. Fine if you get a beer engine and can hide them away though. I guess Speidels are a little ugly as well if you intend to keep them out in the open, but it should be possible build a (chilled) box for either cubitainers or a Speidel and gravity serve from that (with the option of adding a beer engine to the top later and mounting the box lower down).
Refurbed beer engines are cheap in the UK (about $100), but you probably won't find anyone on ebay willing to ship directly to the US. Useful if you have a contact over there you can use though. I intend to try and pick one up for next time I'm over there (to get married), although that will involve shipping it back to the US in my suitcase via my honeymoon. So I still need approval for that (we would also use it for the wedding as well though, so I might get away with it).
