Oh my God. I've spent enough money on this to buy a car for about $3,000.
My biggest army was a 3,500 point Dark Angels Space Marine Army.
I also had 1,500 points of Tau, as well as Daemon Hunters and some Necrons.
Its a really fun game and I absolutely love the modeling aspect of it, thats what always kept me coming back.
But it cost way too much, and over time people stopped playing so I ended up selling all my shit.
I've switched over to playing Necromunda(In the 40k universe, but gangs instead of armies). Its cheaper and I like the gang aspect, its a little more in depth with your squads.
As for guard... Not the best starter army but it will do. Im assuming you have a few Leman Russ' as well as a basilisk. Imp Guard doesn't rely on just one unit, it is effective when all units are used in conjunction with a good/decent strategy. Good luck, and say goodbye to extra money:D