Choosing an instrument is tricky. Please bear in mind that what follows are my opinions based on what I've seen, heard and played.
Neil Brook is an excellent maker. His Wrens are wonderful little gurdies, with affordable cases. I've seen several and played one. They had very light keyboards, and good biting chiens.
I hadn't heard of Helmut Seibert, but from his website I'm impressed. His designs look a little bulky for my tastes, but they look to be fairly good little gurdies. Not having ever played one, I can't say for sure.
I've played a 'top of the range' George Leverett gurdy and it was very poor. A weak sound, economy tuners (always
shell out on good tuners/pegs. It will be worth it), and generally poor construction (e.g. I could see a pile of congealed sawdust through the roughly cut, oversized wheel hole). I hear his work has improved, which would be good, but he's not someone I can recommend until I play a really good instrument of his.
Kurt Reichmann is a good maker. I've played one of his gurdies (belonging to Bridget Haplin on here), and it was well constructed and had a lovely sound. When I tried it the keyboard was rather stiff, but I hear she's sorted that out now.
There are a fair number of second hand instruments around, too. In a month or two I'll be wanting to sell or hire out my Chris Allen symphonie, which is a lovely little instrument. It's perfect for a beginner (it has mechanical geared machine heads, which simplifies tuning a whole lot), if you can bear to wait that long!
Then again, most makers have a fairly long waiting list.
RE number of melody strings: I've rarely ever seen instruments with one melody string. They're usually tiny little 'medieval' models, electric gurdies or ultra budget compact instruments. I would go with two strings.
Hope that helps!