Actual free blueprints are hard to find on the internet. You may want to check eBay if you don't mind paying for them, as I have seen ship blueprints, original postcards and even model kits of auxiliaries there often.
For photos, you can check uboat.net, although you probably know about this site already.
http://uboat.net/allies/merchants/
Also, this warship site has a few pictures of naval auxiliaries which in some cases were similar to merchants of this era.
http://warships.web4u.cz/staty.php?language=E
Sorry I can't be more helpful.
