$1800 for a GP3500 + 3 coils is a great buy! Have seen them still fetching between $2500 & $3000 even if demand is down so that's a good pick up.
Don't know what Ismael charges for mods now but they used to be $275 for the GP series which is a lot cheaper than some modders & he has a good rep so should be a good thing for under $2500.
One thing I would do, & I'm just thinking out loud here, is send it into Minelab before getting it modded. For about $200 they will make sure the alignments ok & upgrade to the latest specs available for the model. That way you would at least know the Minelab end of the detector is ok before modding. Once it's done they won't touch it although there are a few people about that can fix some issues with them, and Ismael could too, but if you blow something like the MCU nobody except Minelab can repair it as they don't have the codes to reprogram.
Ismaels blog on it:
http://www.minelabmods.com/joomla/index.php?option=com_content&task=view&id=135&Itemid=1
