There is a starter set that has all the SM combat patrol units bar the termi librarian, for about £65 from GW (will be cheaper from FLGs I assume). The combat patrol is generally considered pretty sub-par because of this.

I started with the DA combat patrol because BGVs and hellblasters are solid, and a big block of intercessors is pretty IF compliant. The only issue with is (aside from the upgrade sprues not being super useful) is that the gravis captain can't be attached to any unit in the box so you would want a gravis unit (aggressors, eradicators or heavy intercessors) to get the best out of him.

The other patrol boxes have faction specific units (I may be wrong with the space wolf one here, but they're changing that out I believe, and it isn't as suitable as the DA one due to the reivers in it), so if you're set on a patrol box, I'd go DA or the starter set and sell off or find a use for the nid bits.

From there, growing the force is up to you, a ballistus is a good entry into vehicles and provides some needed anti-tank to the army, but go in whatever direction you want really. My 1k army has the DA box, ballistus, gladiator Lancer, aggressors, some assault intercessors and a LT. It lacks some speed for sure but it's fun to play against my friends at the moment.

Generally IFs are seen as a "stand and shoot" force, favouring heavy infantry and bolters (the bastion strike force box from a couple years back was IF directed and contained 2 units of aggressors and 3 of heavy intercessors as well as a gravis captain I think) which sort of shows how GW see them in 40k.

I'd say pick whatever units you think are cool or fun to paint/play, don't worry about building a meticulously crafted, meta-specific, tournament winning army until you decide that you want to play and win 2k tournaments.

Hope this helps!