I use Chief Architect to do layouts/constructions drawings and can handle any permits needing pulled.I have capital that I’m willing to invest.

It's in a city (outside the USA in Mexico) that has seen major growth the last 3-5 years and has great prospects to continue growing thanks to nearshoring and high cost of living across the border in the USA.I spoke to an architect and it looks like a 4-unit building can be built on there.

Find architects or designers, engineers of all types, general contractor, etc. to design the project, and know how to keep the design where it's functional, cost effective, but has appeal.

You need an architect absolutley and that is where you start. 

You could also work with a few professionals like a surveyor, landscape architect and/or engineer to help. 

I recommend you consult with an architect.

I've been an investor and broker for the last 10 years in FL and NC, and specialize in the mountain regions, and beach regions because that's where most of the tourists, and investors who want getaways, invest (certainly there are many other interesting markets to invest however, booming wine country for example..)To add to it, I love the environmentally responsible home building, that's something I've had an interest in for a while, and I've worked with several contractors and architects in the last few years to come up with a useful plan for alternative and environmentally responsible building materials. 

I would reach out to an architect. 

If not please delete this post.Can anyone recommend an architect company who is familiar with ADU and is responsive, organized and creative in problem solving?

Really making sure that you stay on top of that person (contractor, you, architect) on the ball by following up with the city to get that permits done in a timely fashion.