Hi and welcome to the Salty side the TL550 is a good tank to start its big enough to hold a few fish and the lights are good enough to keep most Softies, LPS I even managed to keep some easy to keep SPS in there.
I would definatly go for cured rock if you can there are always people breaking tanks down and selling there matured LR off at half price arounf £5 a kilo flea bay or UR
I personaly am not a fan of adding LR into the rear chambers as the flow in them is not great and you could end up with a nitrate farm in the back but I do know some people do like this
The Maxi-Jet upgrade is a good one in my eyes but is not an essetial if you are getting a second hand tank then its worth considering
The Hydors Koralias are good circulation pumps if i started up a TL550 again i think I would buy 2 x Koralia 1s as you can manage the flow direction alot more but a 2 will do the job I ended up adding a Koralia nano to the tank to increase flow arounf the back of the rocks with 2 x 1`s i would not have needed
Live sand or not to live sand first thing I would say if buying second hand do not keep the sand in the tank it will be full of nitrates etc and it will take months to get them down (personal experience
) I have used both Live and non Live sand little diffrence in my eyes but the Caribsea live sand has a nice mix of fine and course bits which looks a little more natural.
A few other things to consider are you going to buy your RO water or produce your own I started by buying my Saltwater from my LFS but have moved onto a RO unit now I have a bigger tank
Test kits most people I know use the Salferi test kits there are others available but these in general seem the better ones you will need Ammonia, Nitrate, PH and Nitrite to start these tests will show you when your tank is ready for the first critters
There are a few more but you wont need to worry about them for a few weeks.
Another thing to consider is adding chaeto to one of the rear chambers this will extract a lot of the phosphates and nitrates from the tank but will need to be lit.
Also the Blue LEDs in the TL550 where a little lacking in my eyes so I added a strip of waterproof LEDs from flea bay for better moonlighting
once you get the tank make sure all the sponges are out of the back chambers these are quiet deep and once the tank is in place a pain to get out
Lets to take on board with these marines but a very rewarding hobby much better than tropical in my opionion