bill001g :
Pretty much you will not find third party firmware for any DSL...or cable modem.... device because there are licensing restrictions when device use open source software. You can't get the software drivers for the modem part of the router.
This make good software support even more important. This is the key reason you pay a lot for the larger brand routers they tend to fix things very fast. Now if you could load dd-wrt you don't care as much but in your case since you are force to use factory firmware it is critical to get good support.
Performance wise you would have to dig though the databases and find out what cpu and radios the device uses. Pretty much there are only 2 or 3 makers of chips for routers. Routers with the same chip will pretty much perform exactly the same.
Still the wireless market is actually very old tech wise so there is not much difference in the range on any router. They all transmit near the legal power allowed. Your house is actually what affects range much more and there is no way to predict why certain router do or do not work well in a particular house.
I have been going through the website of tenda and it seems that they are using broadcom chipset in there routers. Also looking at there website and products list, it seems that they are manufacturing routers since many years.
As far as ddwrt is considered then as your saying that routers with modem won't support it then I think any brand's ADSL router wouldn't support it. So should I go with more common brands like D Link or should I go with tenda?