If the copper travels 300m from you to the cabinet and your house phone wiring is good and doesn't increase line attenuation you can probably expect 18Mbit-20Mbit line rate in the modem for ADSL2+. However don't be surprised if it's more like 15-17Mbit, phone cable doesn't always fly straight like a crow and generally phone house wiring is bad (daisy chained, lots of jacks, no master filter) in NZ.
Real world throughput of 802.11g (wireless g) is around 20Mbit so wireless to internet speed won't increase with wireless N as the modem line rate will be the bottleneck.
Only real reason to upgrade to 802.11n is to increase throughput between devices inside your network that are connected by wireless.
I think Ragnor has covered most, if not all of your question.
It's true, that 802.11n won't give you much if any advantage over 802.11g when ones primary focus is web access.
The other consideration is that if you have 802.11g devices existing on your network, like old PCs or mobile smart phones, these devices will reduce the available bandwidth of a 802.11n radio anyway and thus you're gained nothing.
Unless you purchase a high-end home modem(Not a big fan of one box does all) it won't come with 1GB Ethernet or dual band 2.4Ghz/5Ghz. I have a 1GB Ethernet NAS (Network Attached Storage) and that's really what dictates _MY_ requirement for high end routers (Sans ADSL) and 300Mbps+ WiFi.
I have a Thomson TG585 V8 with an 802.11n radio and it's doing a good job so far as an access modem. It's passing VPNs and NAT ports and as a 'free' modem it even supports separate routes on the WiFi/Ethernet interfaces which isn't normally available on most modems as they normally operate at level-2 switching.
My TG585 is predominately used as a modem/gateway (Not WiFi) allowing access to my core LAN router(s).
I tend to separate roles, such that an ADSL2 modem does only that with an up-link to a router which supports 1GB Ethernet, 2.4Ghz for mix mode 802.11gn and 5Ghz WiFi.