It is great to see the interest in older well maintained designs. The benefits of multihulls are more than just being "faster". We made the decision for an older well maintained tri for the same financial reasons mentioned here. I think any of them mentioned here are worth taking a good look. I think the Marple's is gone.
Giving that one has the ability to sail larger multihulls and understands the challenges, the issue around size of multihull remains "the weight of payload you wish to carry". You do have to go with a larger multihull waterline length, as compared to a monohull's waterline length to carry the SAME payload.
Cross and Horstman developed a evaluation tool to help would-be owners and builders determine the size of boat they would need. You can go to http://mysite.verizon.net/res78939/id5.html
and request the Cross Evaluation Tool and I think you can contact Ed Horstman to get his tool Ed Horstman Designs
Even if you are not planning on purchasing
those specific designs you can compare what each designer
thinks, albeit their approaches are different, it helps you conceptualize what you need.
So our boat has a waterline length of 38.5 with a LOD of 41. I think she is comparable in payload to LWL 35 and LOD 37 of a monohull
but with immeasurably more room and comfort and with the sail handling qualities of a larger ocean going monohull
. She is no speed demon and moves as well or better than her monohull peers of similar waterline length.
The nice part of considering these boats is they do not, for the most part blow the budget
, and if one is prudent they can come away with a really fine boat, they own