The problem with using a manual pump is that if there are any leaks in your bag, seals, or vacuum fitting then the vacuum will be lost over time without the operator necessarily noticing unless continually tending to the glueup until the glue has set. How sensitive the setup is to vacuum leaks depends a lot on how long the glue will take to set versus how much the clamped loads are spring loaded against that vacuum -- the faster the glue sets and the less the setup works against the clamping pressure the less sensitive the setup will be to minor leaks. In more minor glueups or with faster setting glues it is not unusual to disconnect the vacuum pump after drawing down the vacuum and a short observation period to verify the vacuum is holding.
Most vacuum pump setups will either operate a vacuum pump continuously (the simplest design, but requires a pump that can handle extended runtimes) or will automatically cycle the vacuum pump in and out as needed (but requires more sophistication in the design). Either approach protects against leaks for critical glueups, especially when using slower setting glues, though the latter permits monitoring for any possible leaks once the desired vacuum has been reached and the unit shuts off.
Save for very small vacuum projects, the biggest argument against a manual vacuum pump, as I see it, is the need to quickly draw down the vacuum once everything is in place and the possibility of inadvertently shifting the glueup as you furiously pump away while trying to draw down the vacuum as quickly as possible. You don't need an extremely deep vacuum, but (if memory serves me) you will want to be able to generate around 20-24" of vacuum and that can take a considerable amount of time, longer the larger the bag, especially if your pump has a very limited draw down capacity. Ideally you want to remove the bulk of the air very quickly so that you can then begin the process of actually drawing down the vacuum.
There are two major setups typically used for vacuum presses and those are mechanical vacuum pumps and Venturi "pumps" (which are compressed air powered and require larger air compressors as their power source, but otherwise have no moving parts). Both systems have their strengths and weaknesses, but both will get the job done equally well provided the solution is properly engineered and sized for the task. My preference was for a large Venturi type since I have a large air compressor to power it, combine that with a vacuum tank, a vacuum "pressure" switch, a solenoid valve to switch the air supply on and off as needed plus a check valve, intake filter, and gauges to monitor it all and you have all the fixings for a very capable setup if you want to get into such more deeply.
Otherwise you might dip your toes in the water using a shop vac to pull as much vacuum as you can (to remove the bulk air) and then an inexpensive electric vacuum pump to draw the actual vacuum, but how small a pump you can get away with will depend upon how slowly the glue sets and how big your bag is but it must fully draw its vacuum well before the glue has begun to set, but I suspect the frustration of such a setup might discourage you since everything is a significant compromise. That said, if it is something that really interests you then you may as well invest the couple hundred dollars to buy the parts needed to build a good automatic vacuum pump setup (all are available from eBay or the JoeWoodworker website) -- the real money is in the vacuum bags themselves -- vacuum presses really shine for veneering and complex shapes (particularly where there is complexity in all three dimensions). Otherwise the cheapest solution, by far, for simpler Ben laminations is to simply build both halves of a form and then just clamp everything tightly until the glue has cured -- you can't beat that for price, vacuum pressing simply spares you needing to build the second half of the form.
Whatever you decide, we all wish you the very best and hope to see the photos of your work.
