Dave welcome to the bbs. You have one of the best spring rifles in the HW77K. My advice would be to get back into using and enjoying it! You will find lots of choice in the pcp world. Your choice, as with everything, will clearly depend upon how much you are able to spend. Bear in mind when deciding that you will need charging kit and I would strongly recommend a 12 litre 300bar cylinder. You may consider a hand pump but the main problem is not just the effort required but the difficulty of trapping moisture to prevent it getting into-and ruining-the gun's air reservoir. Finally, buying 'mint used' on this bbs is the best way of getting quality kit at much better prices.
Good luck with your choice.