Have you already decided which codex? There are 5 choices each with a different flavour. Then grey knights and chaos space marines(maybe even sisters of battle) which could be called space marines.
I would buy dark vengence as you'll want the templates, dice and rulebook. This will give some perfectly serviceable bikers and marines, with characters and terminators that can only really be used as Dark Angel space marines. It also gives some cool looking chaos models.
Find some assault on black reach marines, they should be dirt cheap.
Assuming codex space marines:
You need a
HQ and 2 troop choices. I would suggest either
AoBR captain as he is cheap or a librarian as he is flexible and a good start as the
HQ. Your troops are scouts or tactical squads(or bikers).
Realistically you will have at least 2 tactical squads. You get one in assault on black reach and one in dark vengeance.
Tactical squads need some form of transportation. This will be rhino or razorback or drop pod. If buying a rhino buy a razorback and keep the lid unglued.
If going scouts use sniper scouts and Telion, as they will make you happy.
Now this is not the way I would go about doing any of this. I would work in reverse. Read and reread your codex. Write a 1500 point list, post it maybe here. Then when you are happy with it start buying models for it.
1500 points sounds a lot but marines only make sense at larger point values, it is hard to expand 750 points worth of marines.
I find the best way to write a list for marines is to start with the cool options, so the elites, heavy and fast. Then choose troops and
HQ in a format which boosts the other slots.
So you choose the coolest models, say sternguard in a drop pod, then choose tactical marines in drop pod and scouts and a librarian in terminator armour as they work well with the sternguard.
Quickly covering the boxsets. All the boxsets have some models that I dislike. I dislike assault marines by the way.
The best boxset is probably ravenwing. You can not glue on the ravenwing parts and it will give you a good number of bikers. Bikers can be troops though biker armies are hard to use well.
Good luck

enjoy your army(pick a chapter whose fluff/story you love the most)
Peace out, Fletch