WiFi, Gigabit, many firmware options.
AFAIK, DD-WRT doesn't support ADSL modems. OP would have to use a modem and an additional router if he were to go down the DD-WRT route.
wi-fi?
again negligible on a 5MB connection, even connected at G speeds should be sufficient for such a connection.
Gigabit?
again only needed if he is connecting a NAS or server for some sort of media setup, as even 100MB wired will be overkill for his 5MB internet.
Firmware options?
again so long as it works why would you want this?
it just seems stupid to me to spend £110 on a router for a crappy internet connection. i am in a similar situation where I am on 100MB with VM and next month I will be leaving them for Sky (the full package). Now my new Sky broadband will be around 5MB.
I already have a N56U (router and n only mode) and a RT-N16 (for legacy devices and setup in access point mode).
Now the hub sky send out isn't "amazing" and I am sure I would benefit from spending money on a modem which I can use SNR tweaking, etc.
But the fact is, i already have 2 really good routers worth £150+ combined or thereabouts, and I don't really want to spend an additional £100 on another modem/router for little gain as even with SNR tweaking I am only going to gain what a few MB at most and I am only going to stick with Sky for 12 months before moving back to VM.
So I can either sell both routers and buy a ADSL one with SNR tweaking, but then have to sell that in 12 months time to buy a cable router again, this process means I will most likely end up losing £200+ on constantly switching routers.
Since the new Sky broadband is going to be so slow, I no longer need to maximise the connections, so no need for 2 separate routers (1 for N and 1 for legacy). So what I am going to do is use the Sky Hub in modem only mode and use the N56U as a router (all devices) and sell the RT-N16.
This way in a year's time when I switch back to VM i still have the N56U to use.
My point being I do not think it is wise to spend a lot of money on a decent router for a crap connection unless you use a NAS or have a server setup in your home which you use all the time.
If that was the case then I think the OP would know the difference between a cable router and an ADSL one which he doesn't.
