In general, I've found that Hardnose is right. Hard to beat a well built new fence. I understand that it can be hard to justify the cost if you're just starting out and you want to count on cattle to help pay the bills though too. Try to patch up the old as best you can, but from the start
INTENDING that it's only a temporary fix, and then put a single electric wire out in front of it on separate fiberglass or plastic step ins, or with standoffs, is where I'd go, and honestly... but
ONLY temporarily.
HT electric is all I ever install on my place anymore, even if it is through the woods. Three wires for cattle as perimeter, single wire for interior. As long as you keep heat on it as your general practice so that anytime they DO touch it it reinforces their fear of it, they won't bother it for a long time if you lose that heat temporarily. I bought one of these,
Flashing Insulator | Fence Monitor | Insulights , or there's this one from Gallagher too...
Live Fence Indicator – Gallagher Fence. Can see that the fence is operational as you drive by on the road. Might be helpful if you're not there all the time, if you can just drive by and see that it's working and not grounded out or a dead fencer.
I don't/won't use anything but
fiberglass/plastic posts (emphasis again on fiberglass... and NOT plastic, but they'd be better than steel any day), so there's nothing to ground the fence out, and NO insulators to break (you'll find that most of your fence issues will come because of a bad insulator, mounted on a steel post of some kind). I've got a stack of used steel posts... won't use them again. And using the standoff insulators "works", but know that you're putting an electric wire right next to all that "perfectly grounded" fence......... Greg Judy talks alot about not putting an electric wire with old barb wire, EVER, ...clean out that entire fence, and start over. He's got alot of really helpful videos on Youtube. And I've found the same to be true. Even where I HAVE taken out the old fence, alot of times there'll be a buried wire that you miss... and sooner or later, one will show up sticking out of the ground and up against you're hot wire. So now I pretty much prefer to start from scratch, clear the old fenceline, and build it right, with NO barbed wire OR steel posts anywhere near that electric fence, and NO insulators... THEN you have a reliable fence.
And spend the money on a BIG fencer right away... joule rating is how you want to compare them, NOT "miles of fence" ratings. Bigger is always better... Something like 15 and above... you don't have to go crazy, but I'd stay away from anything under maybe 6 joules, even if it was just a barnyard fence I was powering. If they get hit with a jolt they never want to experience again, and associate it with the wire they just hit, they'll not attempt it a second time. Put a "little tickler" on it though, and they'll be testing it all the time. Young starting out friend of mine was trying to get away with solars... most of those are like 1 joule... biggest I've seen on the shelf at the farm store was 3 I think.... he ended up buying a 36 joule StaFix... says it's the best thing he ever did. Had alot of fun watching his goats hit that wire for the first time after continually ignoring his solar charger! When grass and brush get up against the fence and you have a heavy dew, that can take alot of power out of a fence, if you don't have a BIG joule, low impedance fencer on the line. Don't ever "not power" the fence for long periods either, especially through grass and brush and woods. The electricity on the line creates a "charged electrical field" around the wire, and that helps to deter vegetative growth immediately close to the wire (within an inch or so). This will "help" to keep the charge strong.
I'm too old to build 'em twice anymore. When I first started out here, I tried to "save money" on fencing materials, using lighter weight wire and posts, etc. That never saved me ANY money... and really, it cost me alot more than it ever could have saved me. Ended up rebuilding pretty much all of it, and doing it right. I wish I had known then what I know now, and had done it right the first time.