We bought our condo from our landlords after living in it for 18 months. Is the landlord actually interested in selling? Sounds like from your post that there's no indication of that. For us, OTOH, landlord rented it to us 18 months earlier because they were buying a new house and wanted to wait for the market to improve before selling. So we knew their plan was to sell.
We had a great experience, because we were able to deal directly with the landlord (no rental agency to begin with, we rented a different unit in the building before moving into their place so we knew them already as neighbors) so neither side had an agent. We took this into account in the negotiations. Negotiations were done fairly "arms length" in that it was all by phone, with each side presenting an offer or counter-offer and then giving the other side as long as they needed to consider/counter.
Someone asked what's so special about this house? Obviously an important question and you need to do the comps. For us, being very very cautious people, something we may have kind of paid a premium for (I think we paid what the house is worth, but I don't think we got a bargain - it felt win/win if that makes sense) was the chance to "Try before you buy" which is quite rare...it made us feel more confident/emotionally secure when we were deciding how high we were willing to go.
We like being able to call the former owners to ask "where did this shelving come from" or "do you know the paint color in the bedroom" but obviously those aren't things it's worth taking a financial hit for...