A note on fork replacement:

All of the frks you linked to will work. The handling might change if you go from 52 to 50 mm of rake, but that will make the bike more stable. It is also possible that the axle-to-crown measurements on the aftermarket forks are different, but probaby not to the point where it will alter things much at all. It is very unlikely you will notice the difference after the first few minutes of riding.

You can get a reasonably accurate axle-to-crown measurement with a tape measure then compare that to published specs on avaialble forks... again - a few mm longer or shorter is not going to change enough to make any real difference at all. You might notice it feels slightly different when you first get on it, but the head-tube-angle change resulting froma 5mm difference in fork length will be less noticable than having an underinflated tire, or an extra water bottle in your handlebar bag.

A note on selecting a new frame or bike:

The fit is the most important aspect of selecting a new bike. IF it is heavier or stiffer or not enough rack mounts or has bad paint is all unimportant if the fit is wrong, and a far crappier bike will perform better if the fit is better. It sounds like you have no starting point by which to judge what dimensional difference you want in your next bike... you need to find a bike shop that specializes in fitting, pay them to measure you up and provide a reccomendation as to what size and length of bike you should be considering. I agree that a 21" (52cm) is probably the totally wrong size for someone 5'11 - you should probably be on a ~56 - 60 cm instead, which will give you higher handlebars and a much more comfortable layout.

Before you buy a new frame please consider all you will need to adapt your parts to the new frame. Stem and headset type and size, rear axle length (Over Locknut Distance - OLD) are all considerations that might have you replacing parts that will rack up the price for your project until it nears the cost of a whole new bike.

There are a bunch of production touring bikes still on the market. Surly, Kona, Cannondale, Jamis, Salsa, Rocky Mountain, Trek, and many others offer touring models in all sizes that will have you riding faster, longer, and more comfortably than before.