My collection niche is pre-safety Marlins. That's the way they were when when I became familiar with them, and that's how they were when I started buying them. That is pre 1983.
Nothing wrong with the hammer block safeties. Truth be told, I've let a round off more than once dropping the hammer to half-cock. Nothing wrong with the hammer blocks. They can be deleted and a plug inserted to fill the hole, if you insist.
Go over the pics on line carefully. You can request additional photos, if you're serious about a rifle. The serial numbers you need are only the first two digits. Date of manufacture can be calculated from that. But for an 1894c, the range of manufacture is not that long. Obviously you want to distinguish between a JM and a Remington era manufacture--after 2009.
You may want to find out what the present owner has been doing with the rifle, hunting, competition, etc. Maybe it's been sitting in his safe. Since you plan to have it gone over and modified, you may find reassurance in a rifle that was regularly shot. You will know that it works. If it was used in cowboy competitions, the action was likely slicked up and perhaps the springs were lightened. If you're planning to have it Ceracoated, the condition of the metal is less important, as it will be covered. But deep pits are a turn off.
Make sure you look at the finger lever. Examine it for rust caused by a leather wrap or a leather pad. If you're going to replace the lever anyway for a large loop, it doesn't matter.
I wouldn't sweat the accuracy. Nearly all the 1894c's will shoot into one hole at 50 yards with jacketed ammo. Accuracy can be improved with hand loads.
Once you land a rifle, get a spare ejector or two. The seldom break, but they can be tough to find when you need one. Sometimes the seller will throw in an extra, or brass, bullets, or dies. Doesn't hurt to ask. I'd keep that communication to private emails, not attached to the listing.
Likely some one else will jump in here, but the most reliable site is GunBroker. Some of the other sites have scammers trying to sell a fictitious rifle. If the price is too good to be true, it is. Pass on that one. Try to get an inspection period with option to return. Don't buy from a seller with a very low sales count.
Line up a FFL to do the transfer for you before you bid on a rifle. Make sure your FFL will accept a transfer from a private individual, and what ID is required. Be prepared to pay for a transfer on his end also, if your FFL requires it.
Are gun shows an option for you? Sometimes transfers from private sellers can go through without a transfer, it the show is a club function and show admission is restricted to members only. Don't know if CA has those. Small commercial shows are not likely to have a 1894c, but who knows? You could find a private seller walking around with one. Learn to recognize an 1894c by its size and configuration. If you go, get there early. If you have a friend or two who collect firearms, have them keep an eye open for you. Again, think finder's fees. There are two or three dealers at the show I attend regularly who tend to have a selection of Marlins. If I were looking, I'd definitely let them know what I was after.
If you're looking at something on line, you can ask questions about it here.
It's good advice to be patient. Visit as many gun and pawn shops as you can. Let the shops know what you're looking for and that you'll pay a premium or a finder's fee. Same for estate sales and auctions. Yeah, I know, you don't have a lot of time. But you're looking for a rare bird. That being said, have a budget and stick with it. Sounds like you're willing to pay a bit more for the right rifle. That helps.