I've done a fair few engine conversions in the past, for customers and privately (Perkins 6.247 into Austin K9, Bedford 330 turbo into Austin K9, BMC 6/98 into Morris MRA1, Leyland O.400 into Bedford RL, Bedford 330 into Bedford RL, various engines into Landrovers, etc.).
To my mind there are a number of points to consider:
1. How bothered about originality are you?
2. What are you trying to achieve with the conversion (power, speed, reliability, economy, spares availability, replace an unobtainable original engine, etc.)?
3. What's your budget?
4. What engine can you fit on your budget that'll achieve what you want?
5. If your object is economy, how long will it take to recoup the conversion cost particularly if you're using an expensive engine or paying someone to do the job?
6. Will your chosen engine fit in the hole (length, width, height, ancillary positions, crank centreline, etc.)?
7. Will your chosen engine have any compatibility issues (maximum revs / gearing, suitable torque curve, 12V vs 24V, air vs vacuum, etc.)?
8. Are there any foreseeable issues with ancillaries (compatibility, different parts needed, availability, fit, routing of plumbing and electrics, etc.)?
9. Will you have to change anything else to suit the conversion (springs, brakes, transmission etc.)?
9. Do you have the time and resources to do it (if you're doing it yourself) or the money (if you're paying someone).
I'm sure there are a few things I've forgotten, but if you're happy with your answers to these questions, go for it!
Doing the work is straightforward enough if you're sensible, and there are plenty of people to ask questions of thanks to HMVF and other forums. Don't forget that mounting the engine is a relatively small part of the job; what'll take the time is doing the plumbing and ancillaries.
I always try to simplify the job as much as possible; for example, make sure you get all the bits you'll need with the engine (particularly on modern engines with management systems and so on) and use the gearbox that came with the engine (it's usually easier to sort out gearshifts, propshafts and so on rather than making an engine fit a box, though that's not always the case).
Hope this helps.