A 7/8 is plenty for any usual fish in the estuary/harbour setting. A straight piece of mono about 2m long is adequate for most situations. If you're targeting snapper particularly you can go up to 10kg as they have teeth that can abrade the line during a fight (lost a good sized one that way on 4 kg). Otherwise 4-6 kg is plenty for most others without the dental problems, and lighter line is also more suitable for smaller flies if you would use for kahawai, flounder, piper, mullet, trevally, mackeral.

Check out the reel is saltwater "rated" - even so make sure to wash down thoroughly after a saltwater session.

Sinking line could be a mixed blessing. If you are going to fish water deeper than 2m- 3m with a bit of a current then should be OK. THe big thing is learn how fast it sinks so you dont let it sink onto rocks which are likely to have nasty sharp stuff like oysters, barnacles.