I don't think anyone's made a program for backwards conversion yet; you're best bet's going to be getting a default texture pack for 1.5.2, resizing it to 512x, and manually cutting + pasting the textures into their new, stitched together forms.
The pause you're experiencing is just the game loading up all those big texture files, just wait a couple of minutes - it will happen.
As for the black and purple texture, I had the same issue (I think it's something to do with the compression)
To fix, go into the resource pack's folder (the [1.6.2]_R3D.CRAFT_Default_Realism_512x_v0.1.1.zip) and then go to assets>minecraft>textures>blocks and open up the problematic texture in paint.net, photoshop, or really anything that allows for image resizing. In paint.net (that's what I use, anyways) go to Image>Resize and set the Resampling to Nearest Neighbor, check 'By absolute size:', uncheck 'Maintain aspect ratio' and change the width & height to 512x512. Click okay and you're good. The trouble you're probably having is that the texture was 512x511 or something similar. Edit:I just checked the folder; it appears that the sapling texture isn't there yet. I would try copying the texture over from the default pack and putting it into the resource pack manually. Strange that it does that though, the game should replace all missing textures with their default counterparts automatically.
I wait for a while then it stops loading and just puts the default on. It never loads all of the way.
i dont know if this is intentional but the folder that you download for default 64x is just labeled [1. i imagine that the [1 is supposed to be [1.6.2 just like the tother folders. i just wanted to let you know that, it doesn't really matter but it would be good to change.
Default has more default abiding textures e.g. They follow some of the 16x shapes from the original vanilla texture pack.
Smooth doesn't follow the default items as much, instead all lines are smoother and more 'realistic' if you like
About the impact on performance, all textures greater than 16x will have a greater impact on your system's performance than default. I assume since you're asking you know you don't have the most beastly specs. Chances are the higher bit textures will cause your system to stress more.
My suggestion is to try the 256x first and work your way backward. A lot of people can't run the 512x so since you're worried about performance I wouldn't suggest to start there.
If the 256 makes you lag more than you're comfortable with, try the 128x and so on, until you find one that won't make your PC cry.