I'd be a bit peed off with perfume to be honest, it smacks of thoughtlessness to me because it doesn't take much imagination. I'd far prefer something to do with my hobbies or somebody having listened to my likes and dislikes but then I guess my husband has probably guessed that I am not the sort of girl who likes perfume. However, I'd have probably kept quiet and made sure I let him know I had enough perfume for next time.
As for having no choice in which path you take with MIL, I am not surprised that you are struggling and that really needs to be addressed. Are you able to buy the house without MIL? I would suggest that you talk to your DP and tell him you like the idea of buying the house but need more time with him on your own first before your MIL joins you. Meanwhile, her housing will be increasing in value which will give her more of a cushion in old age.
Your MIL may have needs but so do you. Please do not underestimate the effects of having somebody living with you. In an argument she is unlikely to take your side. When she needs taking to the Drs, assisting with personal care, etc., who is going to do that? Once she is with you, it is just accepted by the Health Services that no matter what is wrong with her, she has people at home who will look after her. If you are both working, how will that work?
Is she putting money into the property, If it doesn't work out and she has sold her house, how will you dissolve the partnership? Will you personally have ownership/rights to the house? What if your relationship with your partner doesn't work, who gets what? Apart from the emotional side you really need to see a solicitor to sort out the financial legalities. If you find it difficult to communicate now, it is not going to be any easier if your relationship crumbles.