Short introduction, and question about parts.

So hi, I'm new. I'm looking into building a Prusa i3 clone with a box frame. I'm already at an awkward start because of a limited budget and time, plus I just had to buy a large heated bed/platform. It's 300x300, and I'm want to finish this with as little mistakes as possible. I know I need the timing belts, rods, and bearings, if anyone can help me out, or pm me with advice. This is my first RepRap.

Why would you build a Prusa i3 in a box frame? The big bed should better stay fixed or only move in Z-direction.
Avoid using end supported smooth rods for such a big printer. ( relatively big for a first build )
Look into CoreXY or H-Bot frames made from T-slot extrusions and linear rails, that's the way to go.

I would not build a machine with a bed that large moving in Y. More correctly, I should say I wouldn't do it again. Click on the Son of MegaMax link in my sig below. It is how I built a printer with a 300x300 mm bed moving in Y. Itt has a box frame. Maybe you can get some ideas from it.

For a bed that large/heavy, it is better to move it in Z. The Ultra MegaMax Dominator link in the sig shows how I built a 300x300 mm bed coreXY machine (bed moves in Z). Maybe you can get some ideas from that. I highly recommend the kinematic bed mount for whatever printer design you choose. The belt lifted Z axis is also working well, and I'm designing a partially printable worm gear reducer that will hopefully bring the cost of implementing a belt lifted Z axis in line with a screw driven Z axis.

Neither of the two machines uses or needs autoleveling. Both are solid designs that hold the bed level once set up and don't require adjustment.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

This is good advice, but I'm sticking with the frame made out of plywood idea. Again, time and budget constraints. Some day I'll upgrade to makerbeams, but I won't be using any now. So anyway, no smooth rods then?

The OP specifically mentioned budget constraints, extrusions and linear rails are in no way budget minded.

Rods will work, just not as well as linear rails, for most people they work just fine to be honest. I'd recommend at least 12mm rods though for a printer that size. Actually, I'd recommend not building a printer that size for your first and budget minded printer. Bigger printers need more material, bigger parts and need to be built more rigid, all of which are hard to do on a budget. I'd go for a 200 mm or even 100 mm cube just to learn about how the machines work and still get a decent printer out of it.

I do have to agree with the other two about not use the mendel/i3 design with the bed moving in the y axis. If you're short on time, go for a simple cartesian setup like is used in the makerbots and similar. Learning about and implementing coreXY, deltas and so on to scratch build a printer may be too much for a first printer when you're short on time.

Out of curiosity, what's your budget? It'll help anyone trying to make suggestions to know how much you can spend.

Sorry for the late response, S&H is what set me back for the most part. The rest is my fault, procrastination is a terrible habit, I know. I have definitely considered linear rails as opposed to rods. I may have to bootstrap this by carving wood though.