While I cant give you any brand names, but just make sure the Manufacturer will warranty the product with radiant and install accordingly.
Being in the Radiant business, I would say the long term fix would be to change out your boiler for an energy efficient model. If the tubing is not leaking and is otherwise functioning correctly, you will get better heat from the hydronic systems. The electric is limited to 15 watts/ 50 BTU per square foot per hour.
The hydronic is rated for higher temps and way more BTU's available.
Leveling compound should not be a problem either. It will add to the mass of the floor which may translate into slightly increased response times, but negligible.