First off, welcome to the addiction.
Lets start from the beginning. How much reading have you done on cast bullets? I'm going to assume very little at this point. I strongly suggest you hit the books hard before you fire up a melting pot.
When you say lead of fairly pure (to my knowledge) composition
, I'm guessing you mean it's dead-soft, pure lead. If that's the case, you'll need to alloy it with a little tin and antimony to make it useful for what you're intending to cast for. There are several ways to accomplish this, and I'm only going to touch on it briefly. You can buy certified alloys (here's one place I would recommend: http://www.rotometals.com/Bullet-Casting-Alloys-s/5.htm
). You can buy Wheel Weight ingots from those who smelt. You can get raw wheel weights on your own and handle your own smelting. A lot more can be broken down about all this later. The basic purpose of it is this: pressure tolerances of the alloy you are casting need to be matched with the load you are intending to use.
Lead hardness is affected by alloying it with tin and antimony (antimony hardens the lead allowing it to withstand higher pressures without shearing, tin aids in alloying with antimony and improves casting ability of the alloy). Clip-on wheel weights (the lead ones, obviously) have a percentage of tin and antimony in them. If you know how to measure the hardness, you can estimate the ratios within the alloy to some extent. Understanding your alloys will help you determine what it can be used for.
Lube: I use LLA (Lee Liquid Alox) or 45-45-10 (I'd have to explain this one a bit, but for the moment we'll just call it another tumble lube like Alox) for everything now. Honestly, it's a personal preference. Some guys swear by their Lyman 45, 450, 4500 and hard lubes. Some have really become good at using the tumble lubes. Again, read-read-read. The more you learn about each, the easier you can make an informed decision about what YOU want to use.
You eventually want to cast for rifles and handguns both. GREAT! They're all different depending on how you want to shoot them. IMHO, a gas check on a 9mm is a waste of good metal. But in a .357, it could go either way. I have pretty stout target and defense loads that are plain-based. Where a gas check helps here is to help reduce the possible leading on a softer/expanding bullet firing in the magnum velocity ranges (this gets back to pressure tolerances of alloys). 30-30 can go either way too. '06 leans more toward having a check. Gas checks slip onto the base of a bullet made to accept it. There are ways to check a plain-base bullet, but that isn't something you want to toy with just yet. So decide ahead of time whether you want a checked bullet or not, then get the mold that suits you.
One more thing in case I forgot to mention it: READ--READ--READ! Bullet Casting 101 at the top of the casting forum page is a great place to start.