I would have to agree with others, that in your position I would keep the LP12.
The LP12 is and always has been a very good deck. The days when it was described as the ââ'¬Å"bestââ'¬Â turntable are long behind it, (if it ever was), however I would still place in the top 1% - (better than 99% of the others).
Although I have currently moved away from the LP12, I am speaking from my own experience, as I have owned numerous versions (probably 6 or 7) over the years (last one with a 90??? Serial number). Arms used from the Grace 707 through Ittok II & III and had the use of an Ekos for nearly one year, have also used RB300, Alphason HR100, original Mission 774, Hadcock and Naim Aro. Using many many cartridges.
On the LP12 I would rate the Grace 707 as being far more musical than the RB 300.
If you just want to change then do so, you can always buy another at a later date.
If you want to update, start with the suspension, (usually available on e-bay for £20 or so) plus setting up, word of warning be careful and make sure you use the correct oil if your inner shaft bearing has a white coloured liner.
Then upgrade the power supply when you can afford it, go either Valhalla, again through e-bay, or better still buy an internet copy of the Armageddon.
First thing to do, change the K5, try either a Denon 103 or a half decent MM if you do not have a MC capability (maybe a Goldring).