My thought here is that if you are not wanting to spend $1K on a filtration/salt chlorination system, is to not get the salt chlorinator. It may add some convenience, but depending on what is supplied with the pool it may be a headache - others will need to chime in here due to my lack of knowledge in the area of salt systems.
I just remodeled my very old pool and was wary of spending too much on it - I might move and if I don't I will be replacing it when the liner needs to be replaced. For 12 years a 1 hp pump and S180T (150 lb sand) did very well, although I was then on the pool store merry-go-round. When I remodeled, I was reluctant to buy more than a 150 lb sand filter owing to the equipment pad placement well under a full deck, and so I bought another S180T, works perfectly well since I have to let water out of the pool frequently (rain). I have since moved the equipment pad when I hard plumbed the system with schedule 40 PVC - and hard plumbing is well worth doing for much less than $100 including valves.
I note that the Hayward 1 hp Matrix pump and S180T I bought were about $350. It is my opinion that this would be ok on a 21' pool, but perhaps a 200 lb or 250 lb sand filter would be an improvement in terms of a reduced frequency for backwashing. I agree that a 2 speed Matrix pump would be better in terms of lower electricity consumption, but that would depend also on how you plan to schedule the pump running. I run mine during the day mostly when there are swimmers and because I power a much loved fountain off it - so I am ok with single speed. The 2 speed pump would be around $100 more. I run my 1 speed for 8 hours a day to cover the times people will be swimming (almost every day) plus 2 hours at night just for skimming and circulation and I estimate the electricity at a cost of $40 per month. I use an Intermatic timer which cost around $30 to control the pump.
I am not sure what kind of deal you are getting, but what I often see is that the base configuration is low cost, but then when you want to modify and improve certain features, the cost goes up dramatically. Depending on the package, it may well be best to purchase the non-pool items elsewhere, but that also would depend on if you are installing the pool or the supplier.
Other than this, my advice is to invest in a good pool water test kit, either the TFP100 or Taylor K-2006C ... they are well worth it! Also read all of the pool school articles and related documents because these will certainly help you manage your new pool!
