Keep in mind that twin engines will be twice as much oil, filters, impellers, zincs, coolant, transmission fluid, etc. And when something breaks or needs preventative replacement, you'll want to do both (raw water pump, trans oil cooler, aftercooler, heat exchanger, etc etc.)
I have twins and winterization (which I do 100% myself) is about $1000 every year. This year it was a new battery bank, last year it was injectors and a valve adjustment (again, times two), the year before it was servicing aftercoolers and HX. Next year that maintenance is due again. Oh and the AC circuit board went out ... blah blah blah.
Each of those things is THOUSANDS if the yard is doing it, and even DYI the parts are extremely expensive.
And none of that stuff is the stuff you really want to do on an older boat, like upgrade the electronics, replace the head and permeated hoses, get LED lighting installed, modernize your battery charger / solar, etc.
Not trying to spread fear, but go in with your eyes open. All boats need a lot of love, but older boats even more so. $8k annually (before slip and winter storage, and insurance) seems very appropriate to me. Then add enhancements.
BD