Take a look at this review of the Torqueedo Travel 1003: Torqeedo Travel 1003 Electric Outboard Review | Cruising World
. It implies that at full power it uses 1,000 watts and at medium power (maybe 300 watts) he could push his 24' sailboat to 2.5 kts. At that power and speed his 500 watt hour battery pack would last 3 hours.
So, translate those figures to your boat. You probably want to go faster than 2.5 kts but your hull is 18' not 24'. So maybe with the T running at 300 watts you can go 3-4 kts.
A couple of golf cart batteries (and I have no idea what voltage the T uses, so this is just for discussion) will produce 1,300 watt hours of power if you only discharge to 50%, so that will let you go about 7-8 hrs.
To keep these batteries charged up with solar so you can cruise indefinitely (but only in full sunlight of course) will take about 600 watts of solar panels. That isn't a huge number and should fit on your cabin top.
The foregoing is based a bit of theory, an extrapolation of the review numbers, and a bit of experience in solar panels.
Good luck with your project.