Jesselp, of course the 30-30 isn't the most "flat shooting" of the available deer rifles, but it will sure do the job well. Will it work better than the shotgun? No, only different. You will have womewhat more range, I feel the practical range of the 30-30 is about 200 yards. Not to say that it won't work a lot farther, it's just that the cartridge, range should be limited for the sake of ensuring you make a KILLING, humane shot on the deer, or black bear or whatever. Not to mention, game animals don't always fall down dead at the first shot. sometimes, they run some ways. Sometimes, not. Look at it like this, if you shoot a deer at 50 yards after either stalking it, or while sitting in a stand waiting on it, and it runs, you have from 50 yards, until 200 or maybe a little bit more to continue to make hits on the deer until it falls down, or dies. If however, you make a shot at that 200 yard initial shot and the deer runs, you have to start shooting again from that 200 yards, on farther out until the animal falls. Which do you want to have to do? I know someone might answer here and say 200 yards is pretty short of a range limitation for the 30-30, and sometimes it is. But the thing is that not everyone is a long range guru, and you said yourself that your a little bit inexperienced. That's ok, I am plenty experienced, yet I limit my shots pretty drastically because I like to make sure I drop the animal quickly, plus I don't see as well as I used to, so seeing the deer and making judgements as to horn size and general maturity is a lot harder for me than it might be for you. It's different for everybody. You might turn out to be a dang fine shot with a rifle, and end up with a 300 Win Mag and a fat scope with the capabilities and experience to take shots out to 400 or even 500 yards.
As for that Marlin for $300, sounds like a good enough deal, and barring the gun looking like a beat up hammer, or having been used like a boat paddle for 80 years, I would pay that. You should look the gun over real well, and take a knowledgeable buddy over to look at it with you. That way, you won't get ripped off. Yes, it can happen! Good luck!
Mark