Juwel rekord 600 - inbuilt filter with 4 filter system.
50w heater
Both ok.
This is probably made of silicates, which is not what you want for marine. Aragonite is much better since it helps buffer the pH.
What do you want to keep? A T8 would be fine for fish-only or invert-only but would not be sufficient for even a lot of the less-demanding corals.
Bear in mind you'll need one or more powerheads as well even though I gather the tank has some sort of built-in filter.
Can i use conditioned tap water? How expensive is RO water?
Depends on what your tap water is like. If it's loaded with carbonates, you will not get a good result from salt mixes, leading to really aggrevating pH/KH problems. If you are considering tap, you should have it checked for copper, phosphate, KH, pH, ammonia, nitrite, nitrate, and TDS. To do marine well with tap you really need a KH of no more than 2dKH. High phosphates, nitrate, and so on will give you all sorts of algae problems (trace is ok with phosphates as long as you run phos remover), a really low pH would leave you struggling to keep your tank's pH where it belongs (8.0-8.4). So, there are a lot of hoops to jump through, but it's not impossible. I did it for a number of years since I had tap with a low KH, neutral pH, no measurable nutrients or metals, and a TDS of less than 100ppm. I couldn't keep it up indefinitely though, since the tap quality became unstable; my KH eventually went up a lot (TDS got closer to 200ppm) and I had to get an RODI system. If you want the least hassle for a first tank, I highly recommend going with RO/RODI, since it is a whole lot fewer things to worry about.
For RO pricing you'll just have to ask locally and estimate based on your tank size for water changes and RO top-offs to compensate for evaporation. Also consider that shop RO units are not a 100% certain thing to rely on. If they're going to break for the first time in a decade, it's going to be just when you needed to do an emergency water change, so make sure you have space to store a small reserve supply or have multiple shops within easy reach that will supply water. Obviously in the long run an RO or RODI unit of your own will be cheaper, but it is a reasonably big upfront cost if you aren't sure you're committed to salt yet (and if you're in a small apartment or something, it may be very inconvenient to even hook one up)
