From the letter of the law, there is room to be lenient. However, the best way to go about this would be to have a gentile partner join the purchase, and have him be designated as the owner of the floor which is leased to the church. Alternatively, one can condition with the seller that ownership of the floor which is leased to the church will only take effect after the lease is up, and you will diminish from the purchase price all of the assessed income from the lease to the church until the lease expires, and this lease money will go to the seller [hence being deducted from the purchase price at the time of the purchase].

If none of the above is able to be negotiated, then there is certainly room to be stringent to avoid entering the business deal, although one who goes ahead with it has upon whom to rely.

Unless you know for certain that the area contained Treif remnant, you can consider it clean of any treif substance and simply suffice with washing it off for hygiene purposes. Nonetheless, one who desires to be Mihader in Kashrus and not take any chances [due to his suspicion of the presence of treif on the surface, even though this worry not Halachically mandated], may be stringent.