Dropping the gearbox and changing the input bearing is fairly straight forward, the best option is to use a slide hammer to put it in as sometime its an awkward angle or cant get leverage with a clamp or wood.. Infact when I did an enigne swap recently (due to cracked cylinder / block) I dropped the gearbox out anyway aswell as about 50 times on other Hondas in the past! it is about 14-16 bolts total that hold it all in place mount wise and 5 bolts to hold it on the engine!
You're going to need -
Sort of flat area to work on which allows you to jack the car up SAFELY
A good socket set 8, 10, 12, 14, 17 18 19 32mm (I think thats everything) - 2nd 17 or a 17mm spanner also handy!
Jack and stands
small piece of wood about A4 size
Breaker bar 1-1.5ft ish
pry bar
WD40
Mallet
Optional: Ball joint seperator fork
If you're confident in doing it yourself its very doable! my steps to remove it would be (however I take no responsibility if you follow my directions and injure yourself, property or anyone else for that matter):
leave car on wheels on ground, pop off the cap to show the driveshaft nut (32mm) - break this loose with a breaker bar, leave about 10% of it threaded on and hit with a mallet so you see it physically move back a bit, take the rest of the nut off and leave the splines in- leaving the nut on stops any threads getting damaged and spreads load over the larger 32mm area rather than the 24(?)mm threaded spline
unbolt the castle nut on your steering arm and separate that can be tricky, use plenty of WD40, wire brush and a breaker bar or pry bar. Move this off to left close to the inner arch to stop it getting damaged or moved out of line
jack car up, take off the 17mm nut off the suspension leg and bash the location bolt through, turn the hub towards driver inner arch.
pull the leg out the way towards you over the lower arm towards the front so you can push the remaining bit of the cv joint out of the hub and pull the driveshaft towards front end and down so you can get the drive shaft through the lower fork part. - Put the fork back over the lower arm out of the way!
you now need to get under the car and get a chizel or breaker bar behind the gearbox side of the shaft - it will need you to hammer a chizel between the box and the gearbox joint and wiggle - its very doable with 1 person but a second person to pull that shaft out is recommended (I didnt have anyone helping and smacked my self in the face with a driveshaft)
once this is done your drivers driveshaft is out the way for you to start cracking bolts.
id take off the rad hoses to give yourself a bit more room to work with and if you want it the whole rad - when I was doing it I fitted a triple core racing radiator so it wasnt in anyway!!
take off the starter motor and put it somewhere dry! Tuck the wires out the way so you can get to the various bits you need
you have 2x 8 or 10mm bolts holding a bracket on at the rear of the box below where the starter was which holds in the fluid pipe for the clutch master cyl - unbolt these
two 12 or 14 bolts up front of the gearbox hold in the master cylinder - unbolt these and move the whole lot out the way - careful not to bend the lines or else you need to get new ones!!
start doing all the bolts round the box leaving in 1 top and 1 bottom at a diagonal - I believe these are 14 or 17mm from memory
you have 4 on the left holding the gearbox to the chassis which is again either 14 or 17mm - There are another couple bolts up on the bulkhead left side which holds a horizontal bolt and captive nut in a mount LEAVE THIS IN FOR NOW
3 at the back which I think was a 19mm on the rear mount - These are only really accessable from underside
take your jack with your bit of wood and put it under the gearbox to take the load off the mount and bolts - remove the last 2 bolts on the box - There might be a couple more bolts on the gearbox crud-plate I have forgotten.. I just get under and do it and never pay much attention lol
At this point you're going to either have to remove the spring pin in the gear linkage or just take the bolts out the gear linkage plate under the shifter and pull out the lot which is 4x 12mm - leaving the linkage on is faster but also more work when you need to put it all back in!!
You will need to repeat the driveshaft removal on the passenger side except you can leave the driveshaft in the actual hub and just remove it from the box - it WILL MOST LIKELY LEAK OIL
Take out the main mount bolt on the bulkhead left side mount - I'd leave the 2-3 that go direct into the gearbox on and just remove the bolt that holds it onto the chassis
once this has been done check there are no earths left bolts on the gearbox or bolts left holding the engine and box together and you can let off the jack slightly to a point where you can wiggle it and free it out - This is possibly where a prybar comes in handy
Then you just need to take the gearbox apart
I'd pitch in about how to do a bearing swap but this thread covers it on D-series.org http://www.d-series.org/forums/transmission-alley/14079-diy-tranny-removal-input-shaft-bearing-change.html
2nd hand gearbox's are always iffy and you might be hard pressed to find a D15 one, a D16 is longer ratio and (i think?) has a different size clutch / flywheel