All good Gins start with a good neutral spirit.
If my memory serves me correct when I was working for Seagrams, the Guild specified the 'botanicals' run (last run) to be on 90% or better NGS, when using a Carthead still.
Personally I believe the better the NGS the better the Gin. The yeasty ethyl acetate nose you get from poor spirit detracts from what I believe good London Dry gins should strive for.
I do not believe you will get an outstanding product from steeping, then distilling off. In my experience, holding the botanicals in the vapor path (aka a Carterhead still) is the best way. I have had good success from suspending a large teabag in the pot, above the surface of the boil. Extraction rates are far less so you'll need more botanicals.
As for the amount of botanicals, Nick's comments are good, check out HomeDistiller, or there is a great section in the excellent book "Distilled Spirits Worldwide Conference: Production, Technology and Innovation" by Nottingham Press.
The problem you will find with your botanical is the freshness, or apparent dryness, hence the available level of essential oils. The vast differences we see here in Juniper and coriander, means we have to trial every batch. It can be a real chore to get consistency from batch to batch. One approach is to distill the botanicals sepearatly and blend, as you would a wine from different casks.