Would anyone know why when I go to change gears, it's get stuck coming out of 2nd gear, then there's a massive crunch, sometimes gets a little stuck in 3rd but after a good run on motorway it's so silky?

So to hopefully avoid a massive breakdown, i've got a a recon gearbox with only 69k, new clutch kit and gear selectors. (Should i also get a new flywheel to on safe side)?
How difficult is the job to change everything?