I fell between two sizes, and I chose the larger of the two (58cm). The stem on this was 120mm which was too long, so I changed it to a 100mm which is perfect!
Regarding the SKS P45s, I think you will struggle to get enough clearance over the stock tyres. 32c tyres seem fine. Tips-wise, all I would say is that you need a spacer to clear the front calliper. I ended up buying a cheap multi-pack of M4 and M5 Allen bolts and plastic spacers from Ebay – all sorts of lengths and really useful if the ones supplied with the guards don’t quite fit the frame. In particular, getting the right bolt/spacer combination at the fixing at the chain stay bridge. The pack came with all sorts of nuts and washers too.
Bolt croppers very useful to save having to trim the ends of the stays with a hacksaw.
Will try to post a pic of the front spacer arrangement if anyone’s interested.
Cheers
Mike.