OP- I think you are being both reasonable and unreasonable IMHO.
Being a step parent is obviously hard going at times. And yes, the bed issue is annoying and something you need to tackle with your DP. I understand why the situation played out that way, but you shouldn't have been put out of your bed without your agreement. DP could have slept in her room if needed. I also agree children do need consistent boundaries and some form of discipline- disney parenting helps no-one in the long run. So YANBU to be annoyed that your were kicked out your own bed, without any say in it. I can understand the concern that your DP hasn't (as yet) stuck to any boundaries or rules. You would not be unreasonable to try and discuss this with him- but I think you need to have a think about your role and the way you view this child. The root problem with the bed and lack of discipline lies with your DP and not this little girl and you need to focus more on that and less on her.
I think you are very optimistic in saying "she knows me well enough to be comfortable with me" after meeting her for 5 or 6 times. She is probably not totally comfortable with you TBH, however calm and confident she may come across to you. She may be absolutely 100% fine with the whole situation, but if so she would be in a tiny minority- more likely she's putting on a bolshy front to cover her real feelings up.
The way you talk about her makes me a little sad for her, actually. I'm sure she is testing boundaries with Dad (especially as you say contact hasn't been all that regular) and quite probably doing the same with you too. This is normal and can indeed be somewhat trying- I can only imagine it is even harder in a child you don't know well and haven't built up a bond with. And, yes she does need clear and consistent boundaries- but she also needs love, understanding and perhaps a to be cut a little slack not written off as a "brat" after her first overnight stay with you which did not go perfectly.
There is something of about the tone and the "my time with DP" part that makes it almost sound like you (perhaps subconsciously) see this as rivalry for your DP's time and affection. Perhaps not what you mean or think, but it can come across that way. I also think YABU to be annoyed that she did not appreciate/was not massively grateful regarding how much work and money had gone into rearranging/furnishing your spare room. She is 8- she probably has little idea about the effort that goes into redecorating and furnishing a room. And you shouldn't see it as a favour to her- you were merely doing what is expected of parents (and step-parents).
I suppose what I'm trying to say (in the nicest possible way) is this: you are the adult, she is an 8 year old little girl. A little girl who is quite possibly a bit mixed up/unsure of her own feelings regarding her parent's separation and who hasn't had much contact with her Dad since. She may well also be anxious about staying away from home with Dad for the first time in a while, as well as being in a strange house and bed, and also staying overnight with Dad's new partner who she doesn't know all that well. If you can think of it that way, you might be able to be a bit more empathetic towards her. That is not to say she can be rude or do whatever she wants- but I do think a little more understanding would smooth the way a bit.
You need to try and build a relationship with her in a constructive way, before being to heavy handed with the discipline and rules- there is a balance to be had there and you do need to build a relationship before trying to act as a parent would. Perhaps try to see it from a child's perspective. I say it again- think carefully before ascribing adult emotions/intentions/understanding to her words and actions.
I think you also need to realise that you have choices and she had and has none. By this I mean: you chose her Dad- so you chose to be with a man who has a child and you can seek to make rules and make her do things (not saying there shouldn't be rules BTW- there should be). In addition, you can choose to walk away if you don't like the way things are. This little girl did not choose to have parents who split and did not choose to see her Dad far less often. She did not choose to have you in her life and can't walk away if she doesn't like how things are working out. She can't do anything about the situation at all whether she is happy or profoundly unhappy, or anywhere in-between. Sometimes children express their unhappiness by misbehaving or being unkind or rude to the step-parent or parent. Before dismissing her as a brat/little madam etc consider if any of this may apply?
Sorry if this sounds unkind- it's not meant to be. I think you do need a reality check though.