1 pump could be from $80-400 depends what quality size you need, flow pumps from $40 upwards, skimmers for a tank that size will cost at least $300 all the way up to 1K depending on brand /quality. a marine light could be anything form $250 if you really good at diy to well over 1k being that your tank is 50cm high you will need MH probably at least 2X250W plus supplementary lighting.
2 a sump is best as it hides all your equipment some of which may be large and bulky it also enables you to use the return pump as part of the circulation in your display tank.
3
350l is a really good size tank as the larger the volume the more stable the water parameters so you shouldnt have too much trouble but take your time with planning and research also patience with cycling will go a long way to having a stable tank
personally i haven't found having a marine tank much worse than having african cichlids maintenance wise, but it really depends on how you intend to run the tank you can end up making heaps of maintenance with heaps of equipment and dosing regimes if you want to or you can keep it simple both have their advantages and disadvantages
4 when you need equipment ask in the private trade and exchange some1 may have something you need otherwise trademe , other options for getting cheaper equipment are online overseas and NZ sites but be aware of bad quality equipment theres a lot online thats not even worth looking at, perhaps if you think something within your price range post a thread on here asking opinions about it