Hi Peter, before you frame the room in, you need to consider all the aspects before doing so;running the wires, built-in cabinetry, whether you want to use in wall speakers, ceiling speakers, just regular speakers and etc. It is cheaper in the long run to make those choices before hand. I did most of the work by myself and help with friends. SO that saved a lot of money in terms of the construction, cabinets to hold the audio equipment, laying the slab, carpet, sound treatments and yada yada.
I had a similar situation( basement, large room about the same size, bar area and etc) was to build a dedicated self-contained HT room and then set up a TV for the bar area with a secondary small kitchen. Of course that cost more money because of the fact you are doubling some of the various audio/video components. The reason I did this is because I wanted the best audio/video for my HT and the bar area was secondary. And if you think about it, hanging around the bar and just watching TV casually, you don't need to go over the top with components, at least audio wise.
Doing a combo bar and HT room isn't a big problem, you just will just sacrifice some audio purity just because when you dealing with a large room and all the various objects in it, it affects the audio. But really, sound wise you should be fine. Considering it is a large room and you probably don't want speakers just standing around on the sides and rears, consider ceiling speakers or brackets using regular speakers pointing towards the seating area for the sides and rear speakers.
Room treatments can be simple as carpet and heavy curtains on the walls to sound traps. Figuring out sound traps, you will need somebody that knows what they are doing, which would mean contracting somebody to do that for you. Generally professional audio/video installers have building contractors that work with them who have experience building these kind of rooms/setups.
Audio wise, you will want to spend at least 1/3 of the audio budget on speakers. They have to reproduce the sound. With most of that budget going towards the front mains, center speaker and sub(sub, you can spend less or more depending on the mains and how much bass you want). For surround, you want to get a surround processor(like this for example
http://www.outlawaudio.com/products/975.html. This will be the heart of the system. I prefer separate components because receivers lock you into a certain setup and the amps in receivers are average at best. With separates you have more flexibility in terms of swapping out and upgrading as you go along. You can do separate amps(stereo amp for mains, mono amp for center, powered sub) or go with a good quality 5.1/7.1 amp. In terms of surround, you can start off just doing 5.1 to save some money and always add the side speakers later for 7.1 and considering the size of the room, 7.2(two subs) might be an option.
Buying used is a great option and will save you a ton of money, most new/used stereo places offer a trade in value when you want to upgrade. For example, you "x" amount of money on a used amp and a year later, if you want to upgrade, you get full value of what you paid for it on a trade in. Most important, listen before you buy. Of course, it will sound different from the show room but at least you will have a general idea.
Happy HT, the Prisoner...