Kinda scared about a first build...

[HaMMerHeD]

You may also have some luck with a granite threshold (like this: http://www.marblethresholds.com/Abs...anite-Threshold-4-x36-x5-p/grthabstdb0436.htm) with some adhesive sandpaper. They are nicely machined flat surfaces. You'd stick the sandpaper to the granite and then sand the to-be-joined edged on it at the same time. Once it looks at least a bit level, check every 10 or 15 strokes to see if it's a reasonably good match.

That'll get you into the ballpark of flat at least, with relatively light investment.

 

[sk3ks1s]

To joint an edge with a router, you'll need a bit at least as deep as the wood you are jointing.

Not necessarily. If you get a top bearing flust trim bit, it can be as shallow as you like. The bearing will ride the template until it actually starts riding the wood itself as you keep making passes.
However, for simplicity it would be best to have a bit that is thicker than the wood.
As for sanding to joint, if it's any type of hardwood, get ready for a workout. Not to mention that with standard sanding blocks (4") in length, there's a good chance you will encounter dips and mounds. It's also very easy to inadvertently round or taper of edges.
A router will make life so much easier. It can handle so many tasks. Routing, jointing, leveling, planeing, drilling, cutting, etc.