Oil and Gas permitting in Texas is fairly easy in air permitting terms. Is this is a cryogenic plant with amine, dehy/mole sieve, suction/residue compression, tankage, flare(s), and fugitives? Have access to enough power or will you need a turbine to generate power? If all of that, you are looking at a Standard Permit (either 116.620 or NRSP) and a GOP 514 (if that compression is gas powered, maybe not if electric). The TCEQ has a pretty extensive oil and gas calculation spreadsheet that should allow you to generate calcs for all your sources. It will also have a modeling component which is where it can get tricky. I was actually surprised by the fact that BPA is in attainment so that makes it much easier depending on emissions level. If you go Standard Permit and GOP then public notice shouldn't be required either. Options below the Standard Permit would be a PBR but that would only be for a site more like a booster station or small treating facility. Above a Standard Permit would be the Case by Case and FOP. Those are more complicated and require public notice. Nevertheless if I were you I'd start with the oil and gas calculations spreadsheet and see what level of permitting it suggests. What type of permit does the site currently have and what type of facility?
If you need a consultant, I would definitely go with TriCord in Frisco. I haven't used them, but I worked with those guys and know their work very well. It's not impossible to do it yourself either. You can start with a little research. Find a facility similar to your proposed on, ideally in the same county, and find their air permit application in the TCEQs online database. This will give you a good idea of what makes up an application.
Edit...forgot to address timeline. The NRSP will require a simple notification before you can start construction and a full application within 90 days of operation. 116.620 wont allow construction for 45 days after the application is submitted. GOP is required before start of operation if it's a major source (100 tpy for any pollutant).