First of all let's dispense with this myth that Mercury Magnetics transformers are somehow superior - they're not. But they're great at marketing.

If you want a real solid quality transformer set with great tone, stay with Heyboer and don't throw away your money on MM.
Yes, you could use a higher voltage PT with a 5Y3 rectifier, and get a more saggy tone with roughly the same power level. Would be great for bluesy playing. (Just don't use a Sovtek 5Y3, or you really will get higher DC voltages.

) I suspect that MM tried to sell you on that particular PT because either (a) it was higher priced than a regular MM 18W PT or else (b) they have some excess stock they want to get rid of. Did I also mention, they're good at sales.

I would advise against otherwise trying to aim for higher DC voltages, as it will make the amp sound cleaner as well as somewhat more sterile. Of course maybe you want that kind of a vibe - in which case you would also need to use higher value cathode resistor on the EL84s - possibly around 180 ohms.