I replayed all 3 on Steam in 2018, from a modern perspective the shooting in all 3 really needed upgrading, so no complaints there. Also many of the missions were in desperate need of extra checkpoints, maybe they'll add some more.
The £55 price is hopefully at least in part due to re-licensing ALL the old tracks, including the ones they took away from the PC versions. I doubt I'll buy at full price though, £20 to £30 would be a great deal.
The new, vastly improved draw distance should add a lot to immersion, maybe the best thing about this upgrade other than combat.
GTA 3 original version is super rough around the edges in a lot of areas with a bizarrely designed map and all 3 games have situations where horrible aiming can make missions much harder than they need to be, so all round this should be a really good upgrade. Taking the old games off Steam like they have though is just denying new players a chance to see the history of the franchise.