Spectrapure RO/DI system come with all the necessary tubing and fittings for a normal installation. Yes, you do want a handheld TDS meter such as the TDS-3 for $25 additional.
I would not order replacement filters until they are needed. DI resin has a shelf life and you need to use your system for awhile to determine you DI life so you know when to order a new DI cartridge or resin. My DI lasts almost exactly a year with a 100G reef and a 16G nano both using RO/DI. If I ordered resin it would be exhausted from sitting before I needed it.
Forget the word "stages" even exists. What the ebay and some of the other vendors call 5 or 6 or 7 stages isn't really an improvement and can actually be worse rather than better. You do not want nor need two carbons like many provide or you do not want two or three little dinky horizontal tubes with 6 oz of loose resin bobbing around in them supposedly acting as a DI filter.
Spectrapure and many others have done much research and found todays carbon blocks are much much better than even a few years ago so the old method of multiple carbon actually impedes flow and reduces the efficiency of your membrane due to lower pressure. They have also found a low micron absolute rated sediment filter does a superior job of protecting that carbon block so it can actuallty last up to its 20,000 gallon rating rather than some others 10 or 5 micron nominal rated filters. Think about this, you can see 40 microns with the unaided human eye so what size filter do you want protecting your system, a 0.5 micron or a 10 micron? Which will allow particulates and colloidal materials to pass through and foul the carbons billions of tiny pores and which will protect it?
Then they blend all their DI resins in house based on thousands of hours of bench and real world testing so you get the vesry freshest and best resin on the market. No preblended or prepackaged stuff there.
The biggest thing you need to do once you receive the system is flush the filters to waste for 3-5 gallons before installing the DI cartridge so you don't exhaust it and take the time to measure the waste ratio and trim the flow restrictor for your exact conditions. This is something else most vendors do not provide, a capillary tube flow restrictor for an exact waste ratio, no guessing or ballpark close.
16G Bowfront Nanoreef. 100G SPS dominated reef.