your main area of concern with getting a visit visa for your fiancée is convincing the ECO that she would return before her visa expires, for many not an easy task, the chances might increase if she is working, has property or business but then some people have been lucky in getting a visa with nothing and some who had property a job etc have been refused.
this might give you an idea about a letter of invitation
http://www.londonelegance.com/transp...tionLetter.pdf