How to improve/extend wireless signal/coverage inside large apartment?

Hi,

I have to imprive wireless inside large apartment which actually consists 3 apartments located on 65th floor in Manhattan. Initially my customer was with old cable modem with wireless part which came from TimeWarner Cable, but it was very old and signal was bad - they can get wireless just 30 feet away. So, I replaced it with new wireless router, TRENDNET TEW-633GR which improved wireless coverage, but now enought. The apartment is large, and because they are 3 apartments, very long as distance. Thanks God, they don't need wireless on every room, but I should cover at least a half. Hard wiring is not an option, I have to find another solution.

My first idea was to use powerline adapter and add additional AP somewhere in the middle where my clients library is and his office, but from my experience it is not reliable solution - it might work and then need restart. Also, some home appliences can interfere greatly.

Another way might be wireless extender. I found some models - LINKSYS WRE54G (which support only WEP which is not good in terms of security, my customer will have constatntly visitors and just friends with laptops), BELKIN F5D7132 (WPA/WPA2), D-Link DWL-G710 and so on...

So questions:

1. do you think that wireless extender will help me cover 2-3 more rooms located at least 100 feet away from wireless router?

2. can I use more than one range extender, and in this case will it affect maximum speed (I've red somewhere that with one extender actually I can expect just a half of regular speed which I can get if computer is connected directly to wireless router). If I put wireless router somewhere in the middle of apartments (don't know where actually outlets for cable TV are located, one is in the kitchen, second one is in guest room where the modem is right now and third one is in the library which is located somewhere in the middle of apartments) and put 1 or 2 extenders on both ends, whill it work and what will be the speed in this case? And will all of them use the same SSID as main router?

3. I know I can try some antennas to improve the transmission/reception, or use them with range extender/router, but maybe there is another way to get wireless internet here? I'm not talking about expensive Cisco equipment, they are not going to pay for it, should be something less expensive. My existing router is with 3 antennas, maybe I just need to replace 1 (or maybe all 3) antennas with omni-directional (like D-Link 2.4GHz Omni-Directional 7dBi Indoor Antenna) and it will be enought?

Please note, library is about 100-130ft from both ends of apartment and it is located in the middle, there are some walls and pieces of art in the rooms. The router is currently in the guest room on very end of all three apartments and I will move it in the library.

Lets fing the right solution together. Thanks.