with help of great people in this forum I understand that seamless wireless roaming with multiple APs is not posssible without a wireless lan controller.
That's not totally correct. Even without a WLC the WAPs can operate in autonomous you can get wireless roaming good enough for wireless VoIP phones.
One of the few things to consider are:
1. Signal overlap;
2. Channel assignments;
WLC makes it easier to manage channel assignements and interference.
what is the highest signal strength 802.11n AP / Antenna that I could use in order to have wide coverage with a single AP?
Depends on what you are trying to do. Getting external antennas is a two-edged-sword. Antenna strength is useless. You want to remember that wireless transmission is a two-way-street: Both the WAP and the client must be able to hear each other. If the client can hear the WAP (good) but the WAP cannot hear the client, then that's bad.
What equipment would you recommend?
This is the part when I ask you these questions:
1. What are you trying to do;
2. What is your budget; and
3. What is your timeline for delivery.