A New router seems to be needed to sit behind a BT ADSL Modem/Router.
We used to have a R7000 Netgear, but it has been misbehaving for the last year - using DD-WRT firmware
VPN Client connectivity is necessary - as we use ProtonVPN services and other VPN Servers to access other networks
Internally, we have 3 laptops, 1 NAS service files and media files, and mobile phones within the same network
May I please ask for recommendations in relation to hardware that we may be able to buy at a reasonable price - ~ £ 100
The forum has recommendations for FritzBox 4040 and WRT3200ACM
As a business, I hope you are referring to the BT Business Hub (and not the corresponding Home Hubs) - difference being that the former allows PPPoE pass-through (using the Business Hub as pure modem, not as a router), while the later doesn't offer that functionality (also called bridge mode) in its OEM firmware. A lot of things become much easier and more stable/ reliable if you can terminate your WAN ISP (the PPPoE session) on the OpenWrt router and don't need a double-NAT setup. If you aren't, getting a cheap pure-xDSL-modem might be an alternative.
The WRT3200ACM is the faster router (considerably), but its wireless capabilities are hampered by its EOLed wireless chipset/ firmware/ driver, this makes this very powerful device a hard sell, I certainly won't be able to recommend it.
The Fritz!Box 4040 is a good device and should cope with ADSL/ VDSL easily, VLAN setups are a bit difficult (due to driver limitations which are hopefully about to be resolved with the switch to DSA based drivers in the medium future). Wireless is slower on paper (2x2 vs 4x4 of the wrt3200acm), but in practice this would be its biggest advantage over the wrt3200acm, as its driver/ firmware are still actively maintained and are generally working fine.
I would add the ipq8065 based Netgear Nighthawk x4s/ r7800 or ZyXEL Armor Z2/ NBG6817 to the list of options, as kind of a mediary between the two devices you've mentioned - as they come with more performance than the ipq4018 based Fritz!Box 4040, don't have any issues with VLAN setup and offers good/ well supported 4x4 wireless.
If you are fine with living on the edge, the Belkin RT3200 might be another alternative, mt7622bv based, wifi6 - and 99.99 GBP (it did sell for 79.99 GBP until ~a week ago). OpenWrt support for this device is very new to OpenWrt (master snapshots) and you do have to be a bit more involved and flexible with the initial setup (compared to the ones above which have been supported in stable releases for quite a while), but you'd be rewarded with a pretty capable (but cheap) and brandnew device that would be good for quite a bit more than you're currently planning to throw at it.
Finding and deciding between the perfect device for running OpenWrt (and capable enough to meet your needs, without breaking the bank) isn't always easy and might need a little more attention than you might be familiar with, but it's usually rewarded by a well working setup. Your requirement list above isn't quite sufficient (the most important aspect, WAN speed and expected VPN performance), so it's hard to give advice - therefore I'm basing the above on well supported mid- to highend devices that should service any kind of xDSL line (cable- or ftth would be another question).
Unfortunately,like many, I am cornered by The Virus and forced to work from home; the BT Hub is the home type.
Ideally, i would like a router to sit behind a 'modem' connecting to the internet/cable/fibre which is provided.
I presume that i would have to spend time to have the router flashed and configure to my requirements;
You are correct, I would be doing so - i did this with DD-WRT previously
Is the OpenVPN client configuration/setup available with all of these hardware/routers - we need to have 3 clients concurrently using OpenVPN ?
Based on your suggestions, I am considering the Netgear as the better option - let me check if I can afford it.
Please let me know if you need better clarifications.
OpenVPN (or any other type of VPN, like wireguard or IPsec) is a software feature and can be run on 'any' device - although not all of them provide enough CPU power to max out your WAN speed.
For ADSL (<20 MBit/s), all of the options above should do easily, for VDSL (25-100 MBit/s) you'd need to look at the faster options (ipq806x, mvebu, mt7622bv) - above that, it's getting more difficult. Among the various VPN protocols, OpenVPN tends to be the slowest (most CPU taxing) one. Wireguard or IPsec would be considerably faster and easier on the CPU.