Firstly you need a good trusted team and very good developers! no room for errors. I have developed a few in the past and now they are pretty much know in the crypto industry.

What you need

1. Finance in place

2. Developers starting with even 1 or 2 gradually increasing

3. Your own frame work

4. List of features what makes you different to others why would i use your service over other crypto payment providers. USP Unique selling point

5. Trusted hosting company

Now you need to decide the best route.. Crypto blockchain development for Industrial example API based for websites buying and selling or individual using to send and recieve payments.

DevCosting: from $45,000 Stage one
Building somthing like coinbase average time 9 months which is broken down in stages 3 months + 3 months R&D then another 3 months testing the waters.. Website live real time customer feed backs is very important before heavy marketing. Total 9 Months is fine.

Security Costing: $5k to $10k intial startup
Hosting nodes keeping them entirley seprate from the web application server. You can learn alot by watching IvanOnTech Youtube how important security is.

Marketing Cost $20,000 Minimum:

It all depends how fast you want to be known. cointraffic coinzilla amazing advertising platform will get you know int the market fast. before you even advertise you need to build articles build a marketing team for google keywords youtube content.

Security is very important and must never be taken so lightly and many other things to consider as the list goes on and on.