Hello Dan, welcome to the forum.. As requested above, some photos would really help.
Here are some rough prices to give you a very rough range to work within.
A used but refurbished/rebuilt EFM 520 costs roughly $2000-$3500, depending on year, condition, options etc..
I'm estimating that a rebuilt burner assembly is going to run around $1000-$2000. The wide range is based on the specific burner and your needs
for the auger, brackets to install in your boiler, missing pieces etc..
So, based on the above [very] rough prices,, I would say pay $500 or so for the boiler, If the oil coversion is the EFM conversion kit,, not a home-grown conversion, this oil burner has some value as well, so you may want to up the price by a few hundred dollars.
If the EFM's enclosure is not rusted out around the bottom and all the enclosure pieces are there,, I'd say buy it if you can.. Oil burning should have lessened the normal corrosion from coal ashes,, so maybe it's in good shape.
The serial number for the boiler would help, and whether the cleanout door is round or square, and the measurements of the boiler.. There are quite a few EFM 350's in use, as well as different versions of the 520,, they all look pretty much the same
Below is a photo of some EFM rebuilt burner assemblies and a few boilers.
Contact [via PM] member ' scrapper 23' he rebuilds EFMs... look in the 'Items for sale' in the forum,, he had an EFM 350 for sale, it is sold, but the ad may be informative..
Hope this helps,, Greg L.