TD1 @ £7.50 is well worth it the campaign to level 30 is pretty decent though not everyone will love the end game content and at that price you probably don't get the extra stuff like incursions and underground, etc. which are some of the better experiences post level 30 for coop.
One downside for coop play through the main story though is that if you don't play together (i.e. if someone plays for a few hours while the other is at work, etc.) one person can easily outstrip the other making it a bit difficult to play together until you hit level 30 GS 256 without a bit of an impact on the experience.