The 998 would be a good unit to use....especially if you have one to borrow. You would want to use the standard HDSI transducer that comes with it, and your buddy should be able to help you with mounting the transducer correctly if he has one.
If you search this site for "video out" you should find several discussions about it. It works very well, but the biggest challenge is finding a durable, sunlight visible monitor that doesn't break the bank.
(The only units with dedicated DI crystals are the down imaging only units...I don't think that is what you will be wanting to use.)
Some who do the wreck hunting also build a towfish for the transducer to get it down deeper. This will give you better detail in very deep water. There are several people posting on this forum who are very knowledgable about a towfish, and a search of the site should give you several results also.
This site is one of the best sources of info as there are a lot of divers, wreck, SAR, and treasure hunter who frequent this site and use their Humminbirds in unique ways......you could probably get a big jump start by just spending time search previous posts on here.