I've been working with MODX Revolution since first versions it was released for notable clients like RSCPA and MODX LLC (Company behind MODX) and know the requirements and steps how to proceed with upgrade.
What I've done on top of Revolution vary from full blown applications to simple websites and done maintenance work through these years. What makes me a top notch choice could be that I've contributed code to core of Revolution and also am one of those who know it deep down due to my work porting whole application under other database system.
What I would do regarding this project for safe upgrade is to create staging site on your host (if possible) or copy the site on my local linux server just for the time of the project where I could do the upgrades safely and see if something fails. Unless downtime is not an issue for you during night as I'm way ahead of you on timezones so I could basically work during your night with ease.
If upgrade goes without a glitch the work takes an hour tops to get done. Also need to check if all extras are compatible with the latest Revo version, if there are issues with them it more than like that I can fix them which obviously will cost some hours if unlucky.
Please do contact me through this site and we can discuss more about the site.
Br,
Petri
Ps. If there will be a core issue with Revolution. I got whole MODX team online through own channel.