I can't whole heartedly recommend Marine Power, because I've been waiting over 5 months for my exhaust, but their aluminum manifolds are no more expensive the SBC/BBC ones. You can get LS headers for the same price as you will spend on the Lightnings for your BBC.
As far as accessories, they don't have to be expensive. The flywheel was under $100, motor mount adapters can be had for under $50. You could put a carb on it if you don't want to deal with the cost of efi/computer. I'm using the alternator off my 502, power steering pump off a Chevy Trailblazer, steering reservoir off my 496HO and idler pulleys from a Dodge P/U.
Except for the cost of the computer, what it's costing me to do my LS is no more expensive than if I was doing a SBC or BBC..........and if I needed a computer for the SBC/BBC, then there is hardly any difference in cost.
The LS isn't any more expensive than other builds, it's just that they still seem to be relatively unknown in the boating world, even though the engines been around for 20yrs now.
.......but if you can get up and running with the SBC for this year, then that's what I'd do.............................well............... it's not really what
I would do, but it is the smart thing do.