To unlock the Episode IV Bonus door, you need 8 Gold Bricks.
Use this Super Story to get used to how they work, as Episodes V and VI are going to be harder than this. In terms of getting studs, use Secret Plans, you can easily get a majority of the 100,000 needed from this stage alone. About 60-70,000 will be more than enough. Use the remaining stages to get the rest of the total, only really smash things that are on the path though. In terms of long and difficult stages, Mos Eisley Spaceport is long, but can be completed quickly if you know what you're doing. Death Star Escape will probably give the most trouble, considering the sheer amount of enemies in the stage itself, and will most likely eat up more of your time. Overall though, this one isn't that bad.
Location: Mos Eisley
Time Limit: 5 Minutes
This one is not too bad, but can give some trouble. As a general rule, Boba Fett is the character to go with in these stages, as his gun is overpowered, he can hover, and he's a Bounty Hunter. Blowing up metal objects gives more studs than other things, so try to get those as much as possible. 5 Minutes isn't long, but it is do-able. Run around, shoot and destroy as much as possible, it all respawns eventually anyway, so make loops around the area. Never stay in one place and wait for everything to respawn, you'll fail if you do.
Location: Death Star
Time Limit: 5 Minutes
This one's easy. Pick a Vehicle, preferably one that flies, and shoot everything. Remember to pick if all up though, but you'll find yourself hitting the total very quickly. If a bit behind, use torpedos to hit the posts, they'll drop some good studs.