Rather than take random suggestions, why not look at some folk's tank pictures on this, and other, forums? That will give you some ideas of the possibilities.
1) FW or SW. Freshwater costs less for initial setup, ongoing costs may
be less (depending!), and fish are on average cheaper per "unit".
2) Will you want plants? That drives up the start-up cost some, but plants and lights can always be added later and in stages as you can afford it.
3) What substrate?
a) If you want plants later, then I recommend starting with either a sand or ecocomplete (there are several other very good enriched substrates). I have both, in different tanks, and like both. My 55 gallon's substrate cost more than my tank... If you want big plants (swords), plan on about 2-2.5 lbs of substrate per gallon. No plants, you can use less substrate. If you want plants later, start with 1lb/gallon and add more later if you have to.
b) Color of substrate. Most fish show better/darker colors with a dark substrate. I tend to use black substrates, but that's just me. Lots of folks use well-washed white/off-white play sand or various other colors of substrate. Play sand is cheaper, but you have to wash the heck out of it. You can also research the native fish habitat and match substrate color to where a fish is naturally from. That's often impossible to do, depending on the species mix!
c) The fish mix will also determine the substrate, possibly. Species that constantly, and vigorously, stir up the substrate may love sand, but they also can keep the tank a mess.
4) Filter. Planted tanks, in general, can get away with undersized filters, depending on how you stock. The larger the fish, the larger the filter capacity, generally. Depends on how heavily you stock and the species of fish.
5) Lighting. Depends on if you want plants. Fish generally don't require lights. We like lights to see the fish. Almost any light that you like will work but more light is not always better. Limit light to 7-10 hours/day if possible and use a timer to keep a rhythm. If you want plants, read everything you can on plants and lighting. Lots of info on this forum. More is NOT always better, but more light will absolutely grow more algae than less light!
6) Fish? that's your problem, and a great one to have! Back to suggestion of others and looking around. Fish selection will impact
the planted/non-planted decision. For some fish, plants are just fresh, green, leafy food to be devoured.
Enjoy! You don't have to decide soon. Unfortunately, the tank itself is sometimes the least expensive big component of the aquarium "system".
The suggestion about "active" versus "calm" is very important. It determines a tank's personality and the species selection.