The N73 is a good phone in terms of features, battery life and talk time. Also a good thing with the new Nokias is you can update the phones software via the Nokia website. This helps reduce bugs, freezing, extends battery life - as well as a list of other things.
Good choice.
Who are you with atm? If you want to keep the same number you need to speak to your provider and ask for a PAC code. This enables you to transfer your number to another provider.
If you are with O2 currently just upgrade your account to the N73 + the tariff of your choice.
3 are good if you live in a city as the coverage will be fantastic. If your out in the sticks then don't bother.
E50 is a really good phone. It is the upgrade of the
Nokia 6310. It was/still is a very popular phone for business users. Simple, long battery life etc.
It always easier to deal with the network directly (usually best over the phone) so try not to go to Carphone Warehouse, Phones4u etc.