alright i had a little time today so i looked up some router models well within your price range
Routers:
Netgear N900 -$129 Netgear N600 -$99 Asus RT-N56U -$99 D-Link N750 -$89 D-Link DIR-850L -$129
Any of those routers would be a fine choice, many of which you will find in your local best buy
as for the modem
your options are limited but i did find the Arris TM822G brand new for $130 with free shipping
it would be quite a bit cheaper then its motorola counterpart which hovers around $180 just for the modem/telephone
if you decided on the Arris and got say the Netgear N900 both have free shipping and would put your total investment around $250 you would be rid of that pesky leasing charge and have a much more powerful homenetwork
the Arris TM822G is approved for comcast Xfinity aswell
and to answer your questions earlier
Wireless N - correct!
Dual Band - can be simultaneous 2.4GHz is preferable because 5GHz is almost line of sight the higher the freqency the more interference there is going through objects such as walls
E4 - i believe hes referring to the linksys E4000 series router and tomato is a type of open source firmware that can be installed
QoS - Quality of Service think of it like load balancing it's supposed to on a connection by connection basis ( every connection your computer makes to the outside world which is typically hundreds even thousands ) classify and prioritize. what that does is say your streaming a HD movie and someone else on your wifi starts a simple file download from a website, it's supposed to keep your streaming at a higher priority so your packets are always transferred first as to eliminate buffering,
what i've found with QoS is most routers do not have the resources to do this properly so you might actually lose thoroughoutput by the router being overwhelmed with tasks. i usually do not use this feature
and firmware is like an kind of like an operating system think of it as windows is for your computer a firmware is the base operating system for the router or just about any electronic device even your TV has firmware or cellphone