I've never seen a wireless router without a Ethernet port (you couldn't actually call it a 'router' without one) so you're probably safe to assume it has one

. Quite a lot of opinions on the ISP can be found here, albeit a bit mixed:
http://www.ispreview.co.uk/review/products/244.html
Typically ISP performance can vary a lot depending on where you live, the quality of your phone line and the ISP itself. For £5.99 you shouldn't expect wonders, but remember that price is only applicable to certain areas of the country and if you're not in that location then it will cost £11.99.
There's also the long 18 month contract to consider, so if the service ends up being crap then your options become somewhat limited. The £5.99 service is definitely for basic and lite Internet usage, but it sounds like this would suit your neighbour.
The last thing to be mindful of is PlusNet's not so clearly communicated line rental aspect, which even we can't quite be sure about after having just checked the page again ourselves:
Phone line rental - all of our broadband products require phone line rental at £10.95 a month. Minimum contract length of one month applies to Plusnet Home Phone.
We find this quite confusing because the 'Value' package details clearly state "A BT landline is required for Plusnet Value". But then there are plenty of places where the same page states "Packages from only £5.99 * requires line rental at £10.95 a month".
If in doubt, begin the signup routine, see what the prices are and then don't complete it if you feel uncomfortable with the offer.