Well, at least you've got a grasp on where you are and where you want to go. Can't offer much advise on the toe-clips, I went to clipless pretty quick and the pedals I had before then were just platforms.

The Centurion IronMan is a good solid bike, a friend of mine has one in a 56cm size that he built up from spare parts we had around. I'm with you, I love the traditional steel frame look and I doubt you'll find anyone (in this forum anyways) that will disagree with you. The 58cm is indeed about the right size for you, depending on the length of your inseam. I'm 6' myself but have a bit extra length in the legs so I tend to ride a 58-60cm with a shorter stem. The skinny tires and position take a bit of adjusting to, but stick with it and you'll be a speed deamon in no time. One thing that helps is getting on longer roads where you're not constantly stopping or turning corners. Getting a feel for the motion of the bike on a straight away and being comfortable in that helps everything else I've found. I bought a lower end Centurion myself (several models below the IronMan) and recently completely rebuilt it and renamed it the 'Barracuda Mk.I' - I've yet to get any real milage on it but can't wait for the weather to warm up a tad so I can.

Get some cycling shorts, trust me, have the money or not, you'll thank yourself. I got a pair of the Nashbar shorts for about $20 on sale when I was ordering a few other things. I can't imagine how I rode so long when I was younger in jeans! Hit your local Walmart or athletic department store for a couple inexpensive shirts that are designed to wick moisture away. It's not quite a jersey, but at a fraction of the cost.

Hope someone with many more miles with toe-clips can give you some pointers with those, all the folks here in the classic vintage forum (and BF in general too) are always willing to share their knowledge.