I think everything is alright. It would be nice of you to tell them you're just doing research and are currently not interested in joining, and I'm sure they'd be willing to talk anyway (if not just because they 'think' they can convert you).

As a former missionary, I can tell you that I enjoyed discussing my religion with anyone who was sincerely interested in understanding what it is that we believe. Even if the person was not actually interested in joining the Church, I considered a discussion a better use of time than walking up and down streets knocking on doors. I always looked on discussions with people such as you as planting a seed. The seed might not grow to the point of you converting to my religion, but I would hope that at least it would grow to the point of increasing mutual respect and understanding between people of different faiths. You would have to check with the missionaries in your area though to see if they have time. If they are too busy with people who are considering converting to our religion, then they simply might not have time for someone who is not. In any event, you should tell them up front that you are not actually interested in converting. There is something else that you should keep in mind. There is a very thin line between asking questions with an intent to gain understanding and asking questions for the sake of arguing religion. So, you should be careful of the intent with which you go into such a discussion. As long as you are respectful and show a true interested in understanding, the missionaries will probably be more than happy to talk with you. However, if the missionaries perceive that you are only interested in arguing, then they will probably not come back after the first visit. When a discussion devolves into an argument, then no real learning is going to occur. So, it would just be a waste of time for both you and them to continue.

Personally i think its awesome.. i mean knowledge is a great tool. and what better way then to get it form the mouth of to faithful members whom have decide to give up their "lives" to serve their God for 18 or 24 months? i can testify to you as a convert that is it an amazing feeling to get answers to question that was burns inside you, when other church's could not answer you... It wont hurt, to ask them to visit you, i am sure they would like to meet a friendly person in search of knowledge.