Well, it's really hard to say without knowing what types of projects/work you do. But, I'll make a few generally recommendations.
You simply cannot go wrong with the TS 55 REQ track saw. It's the tool I use the most. Great for all sorts of applications but it really shines at breaking down sheet goods. The cut quality is exceptional, very easy to use and super accurate.
One of the biggest advantages that Festool offers is phenomenal dust collection. So, a HEPA Dust Eextractor would be a great complement to this sander and can be used with other tools, even those that aren't made by Festool. The CT SYS is Fetsool's most compact and portable dust extractor and it's HEPA. It's the same form factor as a Systainer, so easy to stack and go. Great for working on the jobsite for remodeling jobs. The CT MINI/MIDI is a step up in capacity and portability since it has wheels. The CT 26/36/48 are the biggest units and the CT 26 is the most popular.
Festool makes some brilliant sanders. If you're getting the ETS 125 then its best companion could be the Rotex RO 125. This sander is a work horse and very powerful. It has a gear-driven mode that strips material at the speed of a belt sander. Flip back to random orbital mode and it's a finish sander with a 5mm stroke. So, it would complement the finish sanding capabilities of the PRO 5 sander.
Those are a few of my recommendations. If you have questions about a specific tool or tool category, let me know.
Hope that helps,
Shane
Thanks! I tend to lay low and try not to be spammy. So, I just hope in when I see a thread where I can answer questions and be helpful.