Boiler: replace vs. clean (symptoms of cracked heat exchanger?)

I realize "doityourself.com" might not be the best place to ask about opinions regarding professional help, but this is the most knowledgeable place about boilers that I can find on the Internet, so here goes... Boiler: Peerless MI 06 / ~18 years old / ~140K BTU Basic questions (full details/story below the pics): 1) If I have a cracked heat exchanger, is it possible/probable that my boiler just needs a major cleaning vs. replacement? 2) Are the symptoms of a dirty boiler/filter/etc similar to those of a cracked exchanger? (e.g., "soot" residue on floor below unit, flame coming out from below part of unit) 3) Does $1,500 sound like a reasonable major cleaning estimate for a boiler of this type, size, vintage? Attachment 42519 (http://www.doityourself.com/forum/attachments/boilers-home-heating-steam-hot-water-systems/42519-boiler-replace-vs-clean-symptoms-cracked-heat-exchanger-peerless1.jpg)Attachment 42520 (http://www.doityourself.com/forum/attachments/boilers-home-heating-steam-hot-water-systems/42520-boiler-replace-vs-clean-symptoms-cracked-heat-exchanger-peerless2.jpg)Attachment 42521 (http://www.doityourself.com/forum/attachments/boilers-home-heating-steam-hot-water-systems/42521-boiler-replace-vs-clean-symptoms-cracked-heat-exchanger-peerless3.jpg)