My 2008 TranceX came with F120 forks, but you can go a bit longer. I currently have 150mm air Sektors on mine (nice light, plush fork BTW. Am thinking about putting the two spacers in to knock them back to 130mm as I find it a bit slack for UK trail riding. If I was taking it somewhere with uplifts I’d leave it at 150mm.
No idea what frames cost second hand, but they are expensive new when compared with the off-the-shelf bike builds.
The point about the headtube length is spot on. Check-out steerer tube lengths carefully before committing to buying a used fork.