First off, definitely go with Recruit a Friend.

You can start off with 2-boxing.
That gets you a strong foundation.

Two toons would be excellent for quests... not a pain to do collections.
Twice the kill speed for kill quests, faster mob respawns because more toons in the area.
The option of both Tank + Healer, for extremely fast Dungeon Finder; and with that many toons, even if DPS are dorks, they cannot group boot you and will be replaced very easily by the mass of DPS that want tanks/healers.

PvP-wise, two aren't enough to outright one-shot blow people up.
And you do suffer from the slave being on auto follow, the possibility of a split team etc.
But you do have double the firepower.
And take up only 2 slots in the 10/15-man battlegrounds, which is often a huge edge.

It is up to you, whether 2-boxing is enough or you'd want to hold off until you have five.
I two-boxed, without software for the first year and a half, and had a blast.
Was even two boxing literally, with two PC's + two monitors + two keyboards + two mice.
Would have been so much better with multiboxing software...
The only question that matters is.... do you find two boxing fun?

I'll recommend having a Death Knight, Druid, Shaman, Paladin and Priest on both accounts.
That gets you any tank (except Warrior, include that if you want... but hate to box them) from either account, along with several healer options.

Down the road, if you decide to 5-box, when you're more financially able...
Take whichever account has the best geared tank on it.
Call that account A, and the other account B.
Purchase three additional accounts.
And link them like this:
A > C > D
B > E
And then boost BCDE with A.
A will get a bunch of granted levels, and BCDE will all have RAF bonuses.