Your pH is good but at the lower end of the range for Malawis. Generally I would advise 7.8 - 8.2 but 7.6 is acceptable, with a view to raising it slightly (and slowly) either by using buffers or using crushed coral in the filter or as a substrate.
A 4 foot tank is also fine for a Malawi community (assuming the depth & height are ~18 inches +). If you want a natural looking tank then don't listen to the shops - they'll tell you to cram as many fish in as you can! This is a common technique with malawis to dilute the aggression. The idea is that one fish can't single out another if he can't see the wood for the trees... this is not what I'd advise.
When I kept Malawis I started off with a tank full (about 20!) in a 4 foot tank (I inherited the fish with the tank) and sold off the majority of them. In the end I was left with 3 groups of fish - Pseudotropheus Acei, Labidochromis Caeruleus, and Pseudotropheus Saulousi Coral. I found these species to be the least aggressive and the most interesting to watch (Acei group together and if you get a group of 5 or more it's great to watch them following eachother about the tank!)
I'd go for a maximum of 10 fish in a 4 foot tank. I'm afraid I can't say that there won't be any problems - there more than likely will be some aggression and you may have to sell off or give away the culprits or the weaker fish (go with your instinct - if one fish is being hounded by several of the other fish then it's likely that if you removed the bullies then the other fish will start to bully it. There's a sort of heirarchy and if there's one extreme in the tank (either a very weak fish or a very aggressive fish) then it's thrown out of balance. You can reach a balance but it takes a bit of patience. This is why many people tend to go with the overstocking method, but it's much more rewarding to see your fish happy with their surroundings and acting as they would in the wild, rather than being closed in a small space with nowhere to hide.
That brings me onto something else - you'll need lots of rocks for the fish to set out their species and hiding places. I'd recommend 1/3 of the volume of the tanks should be filled with rocks. Ocean rock is good as it has lots of holes in it. I use ocean rock in my breeding tanks but currently I have slate and plant pots in my display tank.
Sand is good as a substrate as some species like to dig, and soft wand is easy for them to shift about and they won't swallow it. Plants-wise you don't really need any, but anubias is a good choice for african rift lakes tanks, and java fern and java moss or moss balls do well also.
Filter wise, if it's twice the volume (per hour) of your tank then that's enough, but I use a 600 l/h filter on my 200 litre tank. The more the better really. I use eheim and JBL filters, but it's up to you what brand you go with. Just make sure you read some reviews first or ask here if you're not sure.
I'll let you in on a secret. I've never bought a new tank, and I've never had a tank leak. I built my most recent tank and stand from scratch (it's in another thread somewhere) so that's an option but probably not one you'd want to take unless it's something you'd enjoy doing and would be prepared for a steep learning curve and a lot of effort.
I get all my tanks off classified websites or auction. If you do take that route then just make sure you view before payment and inspect it thoroughly as I once bought a tank and realised when I got it home that it had a crack along the bottom of the sump section. There's some real bargains to be had if you keep an eye out. I bought a 6 foot tank last year with stand, sump and pipework for £120 delivered :D
If you buy a new tank then you'll be looking at 2 - 4 times the average price you would pay for a second hand one. The Juwel Vision 260 or the Juwel Rio 240 would both suit your needs, and come with good lighting. The Juwel Rio 240 is £120 cheaper (RRP) and roughly the same specs as the Vision so I would go with the Rio personally. You'd just be paying extra for the curved front. I've had a tank with a curved front and I didn't like it. Anyway, these are just examples. I've had Eheim and Clearseal tanks and they have been fine. I'm not sure if Eheim make tanks any more though, and clearseal are budget tanks and don't tend to come with accessories, hood, etc but you can buy them and they usually do work out cheaper than most. I leak tested 25 clearseal tanks that had been sat in a garage for 5 years and they were all water tight.
EDIT: Oops I just noticed you mentioned a RIO 240 in your post. Great minds think alike :D