I don't want to deter you from your endeavors, I also don't know for sure what your plans are with this 350G system so please don't let anything I say hold you back, I want you to succeed.
350G FOWLR System
Filtration (including protein skimmer): $800 - $1,500
Lighting (basic): $300 - $600
Circulation pumps: $200 - $400
Live rock (175-350 lbs): $875 - $2,100
Substrate: $200 - $300
Heaters: $100 - $200
Testing equipment: $100 - $200
Miscellaneous equipment: $500 - $1,000
Total estimated cost for 350G FOWLR (excluding tank and stand): $3,075 - $6,300
350G Reef System
Filtration (including high-end protein skimmer): $1,500 - $3,000
Lighting (reef-capable LED or T5): $1,500 - $3,500
Circulation pumps: $400 - $800
Live rock (175-350 lbs): $875 - $2,100
Substrate: $200 - $300
Heaters: $100 - $200
Testing equipment: $200 - $400
Calcium reactor or dosing system: $500 - $1,000
Miscellaneous equipment: $1,000 - $2,000
Total estimated cost for 350G Reef (excluding tank and stand): $6,275 - $13,300
These numbers are surprisingly cheaper than I expected and you absolutely can buy these things over a long-long period of time like I myself have done. Just trying to show what you'll need and it's rough cost but you can also save some things for last like lighting, reactors and random jibs and jabs.