Personally I always use s/s fittings and if there is access to the inside of the boat I use bolts with washers and nyloc nuts. I was put off cheap marine fittings by numerous failures and near disasters years ago and have never looked back.
As to ESSENTIAL mods:
Top of the list comes a system for attaching everything that you do not want to lose overboard to your boat (sooner or later at worst you are going to turn over in the thing or at best drop something!).
I use thin, strong line (e.g. paracord) attached to one or other of the deck fittings with a tiny s/s springclip at the outboard end (or - less preferably - a line tied to the item with a s/s springclip to attach to a deck fitting).
For securing rods I tie a loop in a small length of thin, strong line, twist it into a figure 8 and then fold it in 2 so that there are 2 circles about the same size as the circle made by touching your finger tip to your thumb tip. I then insert the rod butt into these circles and slide the loops up the rod to the reel seat. I fit the reel so that the reel seat sits between the two circles of line with the crossover of the figure 8 on the other side of the rod (that way the loop cannot slide off the rod and if the loop is small enough it will not interfere with the reel). Then I can clip one of my static lines to the line on the rod to secure it.
This method is quick, cheap and unobtrusive in use: you need one static line with springclip per rod holder and one small loop of line for each rod you want to secure. As opposed to having a line with a clip on it dangling off every rod you own - the solution that I have seen some kayakfishers use.
Oh, and did I say it works - field tested by me!
More or less the same system can be used inside the boat to attach essential things you need to have at hand to the inside lip of the hatch flange (if you look there should be a couple of small holes in it & you can drill more). Things have a habit of sliding about inside the boat but if they are tied by a length of mono to the centre hatch you will always be able to put your hands on them if you need to.
As to other mods - I would forget them until you have experience of what works/doesn't work for you - you now have the essential ones IMO.
However you might want to consider investing time, thought and money at this stage in ensuring that you have on board adequate safety kit and drive spares, tools and the knowledge to use them so that you can be self-reliant on the water with respect to always being able to get back to where you started from even in the event of a drive failure or a 'total loss'. (You can go a long way in a Mirage Drive kayak in weather which would put off a paddler - but if the s and the fan come together and your drive fails would you be able to paddle your boat back into that wind ? N.B. you say yo have the sailing rudder? The sail kit gives you a third means of propulsion and it has saved me number of very long paddles in my Adventure over the years !)
Welcome to the gang !